View source for Blue

'''Blue''' is one of the three [[Primary color|primary colour]]s in the [[RGB color model|RGB (additive) colour model]], as well as in the [[RYB color model|RYB colour model]] (traditional colour theory).<ref>{{Cite book |last=Defonseka |first=Chris |url=https://books.google.com/books?id=c3icDwAAQBAJ&dq=blue+in+ryb+colour+model&pg=PA94 |title=Polymeric Composites with Rice Hulls: An Introduction |date=20 May 2019 |publisher=Walter de Gruyter GmbH & Co KG |isbn=978-3-11-064320-6}}</ref> It lies between [[Violet (color)|violet]] and [[Cyan|cyan]] on the spectrum of [[Physics:Visible light|visible light]]. The term ''blue'' generally describes colours perceived by humans observing light with a dominant wavelength between approximately 450 and 495&nbsp;[[Nanometre|nanometre]]s. The clear daytime sky and the deep sea appear blue because of an optical effect known as [[Physics:Rayleigh scattering#Cause of the blue color of the sky|Rayleigh scattering]]. An optical effect called the [[Physics:Tyndall effect|Tyndall effect]] explains [[Biology:Eye color#Blue|blue eyes]]. Distant objects appear more blue because of another optical effect called [[Aerial perspective|aerial perspective]].

Blue has been an important colour in art and decoration since ancient times. The semi-precious stone [[Chemistry:Lapis lazuli|lapis lazuli]] was used in ancient Egypt for jewellery and ornament and later, in the [[Philosophy:Renaissance|Renaissance]], to make the pigment [[Ultramarine|ultramarine]], the most expensive of all pigments.<ref>{{Cite web |title=Pigments through the Ages - History - Ultramarine |url=https://www.webexhibits.org/pigments/indiv/history/ultramarine.html |access-date=22 April 2023 |website=WebExhibits }}</ref> In the eighth century Chinese artists used [[Chemistry:Cobalt blue|cobalt blue]] to colour fine blue and white porcelain. In the [[History:Middle Ages|Middle Ages]], European artists used it in the windows of [[Religion:Cathedral|cathedral]]s. Europeans wore clothing coloured with the vegetable dye woad until it was replaced by the finer [[Chemistry:Indigo dye|indigo]] from America. In the 19th century, synthetic blue dyes and pigments gradually replaced organic dyes and mineral pigments. Dark blue became a common colour for military uniforms and later, in the late 20th century, for business suits. Because blue has commonly been associated with harmony, it was chosen as the colour of the flags of the United Nations and the [[Social:European Union|European Union]].<ref>Michel Pastoureau, ''Bleu – Histoire d'une couleur''</ref>

In the United States and across Europe, blue is the colour that both men and women are most likely to choose as their favourite, with at least one recent survey showing the same across several other countries, including China, Malaysia, and Indonesia.{{Sfn|Heller|2009|page=22}}<ref>{{Cite web |title=Why is blue the world's favorite color? {{!}} YouGov |url=https://today.yougov.com/topics/international/articles-reports/2015/05/12/why-blue-worlds-favorite-color |access-date=16 April 2023 |website=today.yougov.com |language=en-us}}</ref> Past surveys in the US and Europe have found that blue is the colour most commonly associated with [[Software:Harmony|harmony]], [[Philosophy:Confidence|confidence]], [[Social:Masculinity|masculinity]], [[Philosophy:Knowledge|knowledge]], [[Philosophy:Intelligence|intelligence]], calmness, [[Distance|distance]], [[Infinity|infinity]], the [[Imagination|imagination]], [[Physics:Cold|cold]], and sadness.{{Sfn|Heller|2009|page=24}}

==Etymology and linguistics==
The modern English word ''blue'' comes from [[Social:Middle English|Middle English]] {{Lang|enm|bleu}} or {{Lang|enm|blewe}}, from the [[Social:Old French|Old French]] {{Lang|fro|bleu}}, a word of Germanic origin, related to the Old High German word {{Lang|goh|blao}} (meaning 'shimmering, lustrous').<ref name="Webster 1970">''Webster's Seventh New Collegiate Dictionary'' (1970).</ref> In heraldry, the word ''[[Azure (heraldry)|azure]]'' is used for ''blue''.<ref>{{Cite book |title=A New Dictionary of Heraldry |date=1987 |publisher=Alphabooks/A&C Black |isbn=978-0-906670-44-6 |editor-last=Friar |editor-first=Stephen |location=London |pages=40, 343}}</ref>

In [[Russian language|Russian]], [[Social:Mongolian language|Mongolian]], [[Irish language|Irish]], and some other languages, there is no single word for blue, but rather different words for light blue (Russian: {{Lang|ru|голубой}}, {{Transliteration|ru|goluboy|engvar=gb}}) and dark blue (Russian: {{Lang|ru|синий}}, {{Transliteration|ru|siniy|engvar=gb}}) (see Colour term).

Several languages, including [[Japanese language|Japanese]] and [[Biography:Lakota language|Lakota Sioux]], use the same word to describe blue and green. For example, in [[Social:Vietnamese language|Vietnamese]], the colour of both tree leaves and the sky is {{Lang|vi|xanh}}. In Japanese, the word for blue ({{Lang|ja|青}}, {{Transliteration|ja|ao|engvar=gb}}) is often used for colours that English speakers would refer to as green, such as the colour of a traffic signal meaning "go". In Lakota, the word {{Lang|lkt|tȟó}} is used for both blue and green, the two colours not being distinguished in older Lakota (for more on this subject, see [[Blue–green distinction in language]]).

Linguistic research indicates that languages do not begin by having a word for the colour blue.<ref name="languages">{{Cite podcast |author=Tim Howard |url=https://www.wnycstudios.org/story/211213-sky-isnt-blue |title=Why Isn't the Sky Blue? |website=Radiolab at WNYC Studios |date=20 May 2012 |access-date=27 April 2018 |archive-url=https://web.archive.org/web/20181025072538/https://www.wnycstudios.org/story/211213-sky-isnt-blue |archive-date=25 October 2018 |others=Linguist: Guy Deutscher; Professor: Jules Davidoff}}</ref> Colour names often developed individually in natural languages, typically beginning with [[Black|black]] and [[White|white]] (or dark and light), and then adding [[Red|red]], and only much later – usually as the last main category of colour accepted in a language – adding the colour blue, probably when blue pigments could be manufactured reliably in the culture using that language.<ref name="languages" />

==Optics and colour theory==

The term ''blue'' generally describes colours perceived by humans observing light with a dominant wavelength between approximately 450 and 495&nbsp;[[Nanometre|nanometres]].<ref>{{Cite web |title=Wavelength of Blue and Red Light |url=https://scied.ucar.edu/image/wavelength-blue-and-red-light-image |access-date=25 June 2022 |website=Center for Science Education}}</ref> Blues with a higher frequency and thus a shorter wavelength gradually look more violet, while those with a lower frequency and a longer wavelength gradually appear more cyan. Purer blues are in the middle of this range, e.g., around 470&nbsp;nanometres.

[[Biography:Isaac Newton|Isaac Newton]] included blue as one of the seven colours in his first description of the [[Physics:Visible spectrum|visible spectrum]].<ref>{{Cite web |date=2015 |title=The Science of Color |url=https://library.si.edu/exhibition/color-in-a-new-light/science |access-date=25 June 2022 |website=library.si.edu}}</ref> He chose seven colours because that was the number of notes in the musical scale, which he believed was related to the optical spectrum. He included [[Indigo|indigo]], the hue between blue and violet, as one of the separate colours, though today it would be categorised as blue.<ref>Arthur C. Hardy and Fred H. Perrin. ''The Principles of Optics.'' McGraw-Hill Book Co., Inc., New York. 1932.</ref>

In painting and traditional colour theory, blue is one of the three [[Primary color|primary colours]] (red, yellow, blue), which can be mixed to form a wide [[Gamut|gamut]] of colours (although the modern [[CMY color model|CMY]] model is able to achieve a much wider gamut). Red and blue mixed together form violet, blue and yellow together form green. Mixing all three primary colours together produces a dark brown. From the Renaissance onward, painters used this system to create their colours (see RYB colour model).

The RYB model was used for colour printing by Jacob Christoph Le Blon as early as 1725. Later, printers discovered that more accurate colours could be created by using combinations of cyan, magenta, yellow, and black ink, put onto separate inked plates and then overlaid one at a time onto paper. This method could produce almost all the colours in the [[Spectrum|spectrum]] with reasonable accuracy.

<gallery mode="packed" heights="150px">
File:AdditiveColorMixing.svg|Additive colour mixing. The combination of primary colours produces secondary colours where two overlap; the combination red, green, and blue each in full intensity makes white.
File:Closeup of pixels.JPG|Red, green, and blue subpixels on a [[Engineering:Liquid-crystal display|liquid-crystal display]].
</gallery>

On the [[HSL and HSV|HSV colour wheel]], the complement of blue is [[Yellow|yellow]]; that is, a colour corresponding to an equal mixture of [[Red|red]] and [[Green|green]] light. On a colour wheel based on traditional colour theory (RYB) where blue was considered a primary colour, its complementary colour is considered to be [[Orange (colour)|orange]].<ref>{{Cite web |last=Sandra Espinet |title=Glossary Term: Color wheel |url=http://www.sanford-artedventures.com/study/g_color_wheel.html |access-date=25 June 2022 |website=Sanford-artedventures.com |archive-date=7 September 2008 |archive-url=https://web.archive.org/web/20080907184837/http://www.sanford-artedventures.com/study/g_color_wheel.html }}</ref>

===LED===

In 1993, high-brightness blue LEDs were demonstrated by [[Biography:Shuji Nakamura|Shuji Nakamura]] of Nichia Corporation.<ref name="Nakamura">{{cite journal | title=Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting-Diodes | last1=Nakamura | first1=S. | last2=Mukai | first2=T. | last3=Senoh | first3=M. | journal=[[Physics:Applied Physics Letters|Applied Physics Letters]] | year=1994 | volume=64 | page=1687 | bibcode=1994ApPhL..64.1687N | doi=10.1063/1.111832 | issue=13  |issn=0003-6951}}</ref><ref>{{cite web |last1=Nakamura |first1=Shuji |title=Development of the Blue Light-Emitting Diode |url=http://spie.org/x115688.xml |publisher=SPIE Newsroom |access-date=28 September 2015}}</ref><ref>Iwasa, Naruhito; Mukai, Takashi and Nakamura, Shuji {{US patent|5578839}} "Light-emitting gallium nitride-based compound semiconductor device" Issue date: 26 November 1996</ref> In parallel, [[Biography:Isamu Akasaki|Isamu Akasaki]] and [[Biography:Hiroshi Amano|Hiroshi Amano]] of Nagoya University were working on a new development which revolutionised LED lighting.<ref>{{cite web |date=12 January 2023 |title=Professor Shuji Nakamura was key to the Invention of Blu-Ray Technology |url=https://ssleec.ucsb.edu/news/2023/01/12/professor-shuji-nakamura-was-key-invention-blu-ray-technology |archive-url=https://web.archive.org/web/20230324123538/https://ssleec.ucsb.edu/news/2023/01/12/professor-shuji-nakamura-was-key-invention-blu-ray-technology |archive-date=24 March 2023 |access-date=4 June 2023 |publisher=[[Organization:University of California, Santa Barbara|University of California, Santa Barbara]]}}</ref><ref>{{cite web|url=https://www.nae.edu/128641/Dr-Shuji-Nakamura-|archive-url=https://web.archive.org/web/20190411125926/https://www.nae.edu/128641/Dr-Shuji-Nakamura-|title= Dr. Shuji Nakamura|publisher=National Academy of Engineering|archive-date=11 April 2019|access-date=4 June 2023}}</ref>

Nakamura was awarded the 2006 Millennium Technology Prize for his invention.<ref>[https://www.news.ucsb.edu/2006/012148/2006-millennium-technology-prize 2006 Millennium technology prize awarded to UCSB's Shuji Nakamura]. Ia.ucsb.edu (15 June 2006). Retrieved on 3 August 2019.</ref>
Nakamura, Amano and Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of an efficient blue LED.<ref name="NYT-20141007-DO">{{cite news |last=Overbye |first=Dennis |title=Nobel Prize in Physics |url=https://www.nytimes.com/2014/10/08/science/isamu-akasaki-hiroshi-amano-and-shuji-nakamura-awarded-the-nobel-prize-in-physics.html |date=7 October 2014 |work=The New York Times }}</ref>

===Lasers===
{{main|Physics:Blue laser}}

[[Physics:Laser|Laser]]s emitting in the blue region of the spectrum became widely available to the public in 2010 with the release of inexpensive high-powered 445–447&nbsp;nm [[Engineering:Laser diode|laser diode]] technology.<ref name="laserglow">{{Cite web |title=Laserglow – Blue, Red, Yellow, Green Lasers |url=http://www.laserglow.com/GPO |url-status=live |archive-url=https://web.archive.org/web/20110916051206/http://www.laserglow.com/GPO |archive-date=16 September 2011 |access-date=20 September 2011 |website=Laserglow.com}}</ref> Previously the blue wavelengths were accessible only through DPSS which are comparatively expensive and inefficient, but still widely used by scientists for applications including [[Optogenetics|optogenetics]], [[Physics:Raman spectroscopy|Raman spectroscopy]], and [[Particle image velocimetry|particle image velocimetry]], due to their superior beam quality.<ref name="laserglow2">{{Cite web |title=Laserglow – Optogenetics |url=http://www.laserglow.com/page/optogenetics |url-status=live |archive-url=https://web.archive.org/web/20110915023159/http://www.laserglow.com/page/optogenetics |archive-date=15 September 2011 |access-date=20 September 2011 |website=Laserglow.com}}</ref> Blue [[Engineering:Gas laser|gas laser]]s are also still commonly used for [[Holography|holography]], [[Biology:DNA sequencing|DNA sequencing]], [[Physics:Optical pumping|optical pumping]], among other scientific and medical applications.

==Shades and variations==
{{Main|Shades of blue}}
[[File:Shades_of_blue_labeled_with_HSL,_RGB,_and_web_color_names.svg|thumb|alt=Shades of blue labeled with HSL, RGB, and web color names.|Various [[shades of blue]] labeled with HSL, RGB, and web color names.]]
Blue is the colour of light between [[Violet (color)|violet]] and [[Cyan|cyan]] on the [[Physics:Visible spectrum|visible spectrum]]. Hues of blue include indigo and [[Ultramarine|ultramarine]], closer to violet; pure blue, without any mixture of other colours; Azure, which is a lighter shade of blue, similar to the colour of the sky; Cyan, which is midway in the spectrum between blue and [[Green|green]], and the other blue-greens such as [[Turquoise (color)|turquoise]], [[Teal|teal]], and [[Aquamarine (color)|aquamarine]].

Blue also varies in shade or tint; darker shades of blue contain black or grey, while lighter tints contain white. Darker shades of blue include ultramarine, [[Chemistry:Cobalt blue|cobalt blue]], [[Navy blue|navy blue]], and [[Chemistry:Prussian blue|Prussian blue]]; while lighter tints include sky blue, [[Azure (color)|azure]], and [[Egyptian blue]] (for a more complete list see the List of colours).

===As a structural colour===

In nature, many blue phenomena arise from structural colouration, the result of interference between reflections from two or more surfaces of thin films, combined with refraction as light enters and exits such films. The geometry then determines that at certain angles, the light reflected from both surfaces interferes constructively, while at other angles, the light interferes destructively. Diverse colours therefore appear despite the absence of colourants.<ref name="ExeterIridescence">{{Cite web |date=September 1998 |title=Iridescence in Lepidoptera |url=http://emps.exeter.ac.uk/physics-astronomy/research/emag/themes/natural-photonics/iridescenceinlepidoptera/  |archive-url=https://web.archive.org/web/20140407082122/http://emps.exeter.ac.uk/physics-astronomy/research/emag/themes/natural-photonics/iridescenceinlepidoptera/ |archive-date=7 April 2014 |access-date=27 April 2012 |website=Natural Photonics (originally in Physics Review Magazine) |publisher=University of Exeter}}</ref>

==Colourants==

<gallery mode="packed" heights="100px">
File:Egyptian blue.jpg|[[Egyptian blue]]
File:Cobalt Blue.JPG|[[Chemistry:Cobalt blue|Cobalt blue]]
File:Bleu phtalo.jpg|[[Chemistry:Copper phthalocyanine|Copper phthalocyanine]]
File:YInMn_Blue_-_cropped.jpg|YInMn blue
File:Prussian blue.jpg|[[Chemistry:Prussian blue|Prussian blue]], Fe{{su|p=III|b=4}}[Fe{{su|p=II}}([[Company:Cyanide|CN]]){{su|b=6}}]{{su|b=3}}, is the blue of [[Blueprint|blueprint]]s.
</gallery>

===Artificial blues===
[[Egyptian blue]], the first artificial pigment, was produced in the third millennium BC in Ancient Egypt. It is produced by heating pulverised sand, copper, and [[Software:Natron|natron]]. It was used in tomb paintings and funereal objects to protect the dead in their afterlife. Prior to the 1700s, blue colourants for artwork were mainly based on lapis lazuli and the related mineral ultramarine. A breakthrough occurred in 1709 when German druggist and pigment maker Johann Jacob Diesbach discovered [[Chemistry:Prussian blue|Prussian blue]]. The new blue arose from experiments involving heating dried blood with iron sulphides and was initially called Berliner Blau. By 1710 it was being used by the French painter Antoine Watteau, and later his successor Nicolas Lancret. It became immensely popular for the manufacture of wallpaper, and in the 19th century was widely used by French impressionist painters.<ref>Michel Pastoureau, ''Bleu – Histoire d'une couleur'', pp.&nbsp;114–16</ref> Beginning in the 1820s, Prussian blue was imported into Japan through the port of Nagasaki. It was called ''bero-ai'', or Berlin blue, and it became popular because it did not fade like traditional Japanese blue pigment, ''ai-gami'', made from the dayflower. Prussian blue was used by both Hokusai, in his wave paintings, and Hiroshige.<ref>Roger Keyes, ''Japanese Woodblock Prints: A Catalogue of the Mary A. Ainsworth Collection'', R, Allen Memorial Art Museum, Oberlin College, 1984, p. 42, plate #140, p. 91 and catalogue entry #439, p. 185. for more on the story of Prussian blue in Japanese prints, see also the website of the Victoria and Albert Museum, London.</ref>

In 1799 a French chemist, Louis Jacques Thénard, made a synthetic cobalt blue pigment which became immensely popular with painters.

In 1824 the Societé pour l'Encouragement d'Industrie in France offered a prize for the invention of an artificial [[Ultramarine|ultramarine]] which could rival the natural colour made from lapis lazuli. The prize was won in 1826 by a chemist named Jean Baptiste Guimet, but he refused to reveal the formula of his colour. In 1828, another scientist, Christian Gmelin then a professor of chemistry in Tübingen, found the process and published his formula. This was the beginning of new industry to manufacture artificial ultramarine, which eventually almost completely replaced the natural product.<ref>Maerz and Paul (1930). ''A Dictionary of Color'' New York: McGraw Hill p. 206</ref>

In 1878 German chemists synthesised [[Chemistry:Indigo dye|indigo]]. This product rapidly replaced natural indigo, wiping out vast farms growing indigo. It is now the blue of blue jeans. As the pace of [[Chemistry:Organic chemistry|organic chemistry]] accelerated, a succession of synthetic blue dyes were discovered including Indanthrone blue, which had even greater resistance to fading during washing or in the sun, and [[Chemistry:Copper phthalocyanine|copper phthalocyanine]].
<gallery mode="packed" heights="200px">
File:The Blue Boy.jpg|''The Blue Boy'' (1770), featuring lapis lazuli, indigo, and cobalt colourants,<ref>{{Cite web |title=Eight blue moments in art history |url=https://www.tate.org.uk/context-comment/blogs/eight-blue-moments-art-history  |archive-url=https://web.archive.org/web/20181016130010/https://www.tate.org.uk/context-comment/blogs/eight-blue-moments-art-history |archive-date=2018-10-16 |access-date=2018-10-16 |publisher=The Tate}}</ref>
File:Great Wave off Kanagawa2.jpg|''The Great Wave off Kanagawa'' illustrates the use of [[Chemistry:Prussian blue|Prussian blue]]
File:Indigoproduktion BASF 1890.JPG|A synthetic indigo dye factory in Germany in 1890.
</gallery>

===Dyes for textiles and food===
<ref name="Ag">{{Cite journal |last1=Newsome |first1=Andrew G. |last2=Culver |first2=Catherine A. |last3=Van Breemen |first3=Richard B. |year=2014 |title=Nature's Palette: The Search for Natural Blue Colorants |journal=Journal of Agricultural and Food Chemistry |volume=62 |issue=28 |pages=6498–6511 |doi=10.1021/jf501419q |pmid=24930897|bibcode=2014JAFC...62.6498N }}</ref> Woad and [[Biology:Indigofera tinctoria|true indigo]] were once used but since the early 1900s, all indigo is synthetic. Produced on an industrial scale, indigo is the blue of blue jeans. Blue dyes are organic compounds, both synthetic and natural.

For food, the triarylmethane dye [[Chemistry:Brilliant blue FCF|Brilliant blue FCF]] is used for candies. The search continues for stable, natural blue dyes suitable for the food industry.<ref name=Ag/>

Various [[Biology:Raspberry|raspberry]]-flavoured foods are dyed blue. This was done to distinguish [[Biology:Strawberry|strawberry]], [[Biology:Watermelon|watermelon]] and [[Biology:Raspberry|raspberry]]-flavoured foods.<ref>{{Cite web |date=2023-06-21 |title=You Need To Know The Truth About Blue Raspberry |url=https://www.delish.com/food-news/a44272957/what-is-blue-raspberry/ |access-date=2024-10-22 |website=Delish |language=en-US}}</ref>  The company ICEE used Blue No. 1 for their blue raspberry ICEEs.

<gallery mode="packed" heights="150px">
File:Blue Raspberry Frozen Yogurt with White Chocolate Chips, Coconut, and Cherries.jpg|Blue raspberry frozen yogurt with white chocolate chips, coconut, and cherries.
File:Blue haribo jelly beans.jpg|Blue haribo jelly beans. Raspberry flavour.
File:2019-11-09 19 29 30 A blue raspberry-flavored Jolly Rancher Hard Candy Stix after being unwrapped in the Dulles section of Sterling, Loudoun County, Virginia.jpg|A blue raspberry-flavoured Jolly Rancher Hard Candy Stix.
File:Indigo skeletal.svg|Chemical structure of [[Chemistry:Indigo dye|indigo dye]], a widely produced blue dye. [[Engineering:Jeans|Blue jeans]] consist of 1–3% by weight of this [[Chemistry:Organic compound|organic compound]].
File:C.I. Acid Blue 9.svg|Chemical structure of C.I. Acid Blue 9, a dye commonly used in candies.
</gallery>

=== Pigments for painting and glass ===
{{Anchor|main pigment}}Blue pigments were once produced from minerals, especially [[Chemistry:Lapis lazuli|lapis lazuli]] and its close relative [[Ultramarine|ultramarine]]. These minerals were crushed, ground into powder, and then mixed with a quick-drying binding agent, such as egg yolk (tempera painting); or with a slow-drying oil, such as [[Biology:Linseed oil|linseed oil]], for oil painting. Two inorganic but synthetic blue pigments are [[Cerulean|cerulean blue]] (primarily cobalt(II) stanate: {{chem2|Co2SnO4}}) and [[Chemistry:Prussian blue|Prussian blue]] (milori blue: primarily {{chem2|Fe7(CN)18}}). The chromophore in blue [[Chemistry:Glass|glass]] and glazes is [[Chemistry:Cobalt|cobalt]](II). Diverse cobalt(II) salts such as cobalt carbonate or cobalt(II) aluminate are mixed with the silica prior to firing. The cobalt occupies sites otherwise filled with silicon.

===Inks===
[[Chemistry:Methyl blue|Methyl blue]] is the dominant blue pigment in inks used in pens.<ref>{{Cite book |last1=Placke |first1=Mina |title=Ullmann's Encyclopedia of Industrial Chemistry |last2=Fischer |first2=Norbert |last3=Colditz |first3=Michael |last4=Kunkel |first4=Ernst |last5=Bohne |first5=Karl-Heinz |year=2016 |isbn=978-3-527-30673-2 |pages=1–12 |chapter=Drawing and Writing Materials |doi=10.1002/14356007.a09_037.pub2}}</ref> [[Blueprint]]ing involves the production of [[Chemistry:Prussian blue|Prussian blue]] in situ.

===Inorganic compounds===
{{multiple image
 | align             = <!-- right (default), left, center, none -->
 | total_width       = 300
 | perrow            = 3
 | header            =

| image1            = Copper sulfate.jpg
 | alt1              = 
 | caption1          = CuSO<sub>4</sub><sup>.</sup>5H<sub>2</sub>O

| image2            = Cobaltous chloride anhydrous.jpg
 | alt2              = 
 | caption2          = [[Chemistry:Anhydrous|Anhydrous]] cobalt(II) chloride

| image3            = Vanadyl sulfate.jpg
 | caption3          = [[Chemistry:Vanadyl sulfate|Vanadyl sulphate]]
}}
Certain metal [[Physics:Ion|ion]]s characteristically form blue solutions or blue salts. Of some practical importance, [[Chemistry:Cobalt|cobalt]] is used to make the deep blue glazes and glasses. It substitutes for [[Chemistry:Silicon|silicon]] or [[Chemistry:Aluminium|aluminium]] ions in these materials. Cobalt is the blue [[Chemistry:Chromophore|chromophore]] in [[Engineering:Stained glass|stained glass windows]], such as those in Gothic cathedrals and in Chinese [[Chemistry:Porcelain|porcelain]] beginning in the Tang dynasty. [[Chemistry:Copper#Compounds|Copper(II)]] (Cu<sup>2+</sup>) also produces many blue compounds, including the commercial [[Biology:Algaecide|algicide]] [[Chemistry:Copper(II) sulfate|copper(II) sulphate]] (CuSO<sub>4</sub><sup>.</sup>5H<sub>2</sub>O). Similarly, [[Chemistry:Vanadyl ion|vanadyl]] salts and solutions are often blue, e.g. [[Chemistry:Vanadyl sulfate|vanadyl sulphate]].

==In nature==

===Sky and sea===
 
When sunlight passes through the atmosphere, the blue wavelengths are scattered more widely by the oxygen and nitrogen molecules, and more blue comes to our eyes. This effect is called [[Physics:Rayleigh scattering|Rayleigh scattering]], after Lord Rayleigh and confirmed by [[Biography:Albert Einstein|Albert Einstein]] in 1911.<ref>{{Cite web |title=Why is the sky Blue? |url=http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html |url-status=live |archive-url=https://web.archive.org/web/20151102085211/http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html |archive-date=2 November 2015 |website=ucr.edu}}</ref><ref>{{Cite journal |last=Glenn S. Smith |date=July 2005 |title=Human color vision and the unsaturated blue color of the daytime sky |url=http://www.patarnott.com/atms749/pdf/blueSkyHumanResponse.pdf |url-status=live |journal=American Journal of Physics |volume=73 |issue=7 |pages=590–597 |doi=10.1119/1.1858479 |bibcode=2005AmJPh..73..590S |archive-url=https://web.archive.org/web/20110715050855/http://www.patarnott.com/atms749/pdf/blueSkyHumanResponse.pdf |archive-date=15 July 2011 |quote=Near sunrise and sunset, most of the light we see comes in nearly tangent to the Earth's surface, so that the light's path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red. Therefore, when looking at the sunset and sunrise, the colour red is more perceptible than any of the other colours.}}</ref>

The sea is seen as blue for largely the same reason: the water absorbs the longer wavelengths of red and reflects and scatters the blue, which comes to the eye of the viewer. The deeper the observer goes, the darker the blue becomes. In the open sea, only about 1% of light penetrates to a depth of 200&nbsp;metres (see [[Earth:Underwater|underwater]] and euphotic depth).

The colour of the sea is also affected by the colour of the sky, reflected by particles in the water; and by [[Biology:Algae|algae]] and plant life in the water, which can make it look green; or by sediment, which can make it look brown.<ref>{{Cite web |last=Anne Marie Helmenstine |title=Why Is the Ocean Blue? |url=http://chemistry.about.com/od/waterchemistry/f/why-is-the-ocean-blue.htm |url-status=live |archive-url=https://web.archive.org/web/20121118203351/http://chemistry.about.com/od/waterchemistry/f/why-is-the-ocean-blue.htm |archive-date=18 November 2012 |website=About.com Education}}</ref>

The farther away an object is, the more blue it often appears to the eye. For example, mountains in the distance often appear blue. This is the effect of atmospheric perspective; the farther an object is away from the viewer, the less contrast there is between the object and its background colour, which is usually blue. In a painting where different parts of the composition are blue, green and red, the blue will appear to be more distant, and the red closer to the viewer. The cooler a colour is, the more distant it seems.{{sfn|Heller|2009|p=14}} Blue light is [[Physics:Rayleigh scattering|scattered more]] than other wavelengths by the gases in the [[Astronomy:Atmosphere|atmosphere]], hence our "blue planet".
<gallery mode="packed" heights="200px">
File:Top of Atmosphere.jpg|Earth's blue [[Earth:Halo (optical phenomenon)|halo]] when seen from space
File:Aerial perspective 1.JPG|Another example of [[Physics:Rayleigh scattering|Rayleigh scattering]]
File:LightningVolt Deep Blue Sea.jpg|The sea
</gallery>

<!-- tangential, almost irrelevant===Astronomy===
[[Astronomy:Blue giant|Blue giant]]s are hot and luminous [[Astronomy:Star|star]]s with surface temperatures exceeding 10,000&nbsp;K. The largest [[Astronomy:Blue supergiant star|blue supergiant star]]s are extremely massive and energetic, and are usually unstable. They are generally short-lived, either exploding in a [[Astronomy:Supernova|supernova]] or periodically shedding their outer layers to become [[Astronomy:Red giant|red giant]]s.
-->

===Minerals===
<gallery mode="packed" heights="150px">
Lapis-lazuli hg.jpg|Lapis-lazuli
Azurite - New Nevada Lode, La Sal, Utah, USA.jpg|[[Chemistry:Azurite|Azurite]]
Natural ultramarine pigment.jpg|Natural [[Ultramarine|ultramarine]] pigment
Logan Sapphire SI.jpg|Logan sapphire
</gallery>
Some of the most desirable gems are blue, including [[Chemistry:Sapphire|sapphire]] and [[Chemistry:Tanzanite|tanzanite]]. Compounds of copper(II) are characteristically blue and so are many copper-containing minerals.
[[Chemistry:Azurite|Azurite]] ({{chem2|Cu3(CO3)2(OH)2)}}, with a deep blue colour, was once employed in medieval years, but it is unstable pigment, losing its colour especially under dry conditions. [[Chemistry:Lapis lazuli|Lapis lazuli]], mined in Afghanistan for more than three thousand years, was used for jewellery and ornaments, and later was crushed and powdered and used as a pigment. The more it was ground, the lighter the blue colour became. Natural [[Ultramarine|ultramarine]], made by grinding lapis lazuli into a fine powder, was the finest available blue pigment in the Middle Ages and the [[Philosophy:Renaissance|Renaissance]]. It was extremely expensive, and in [[History:Italian Renaissance|Italian Renaissance]] art, it was often reserved for the robes of the Virgin Mary.

===Plants and fungi===
<gallery mode="packed" heights="150px">
File:Blaue Primeln.JPG|''Primula acaulis''
File:Ipomoea August 2007-1.jpg|Morning glory (''Ipomoea acuminata'')
File:Vaccinium corymbosum Beeren.jpg|''[[Biology:Vaccinium corymbosum|Vaccinium corymbosum]]''
File:Vinca minor Nashville.jpg|''[[Biology:Vinca minor|Vinca minor]]''
File:Delphinium denudatum 1.jpg|Blue Delphinium flower
Lupinus-pilosus-2015-Zachi-Evenor-cropped02.jpg|Blue Lupme flower ''[[Biology:Lupinus pilosus|Lupinus pilosus]]''
File:Lactarius indigo 48568 edit.jpg|''[[Biology:Lactarius indigo|Lactarius indigo]]''<ref>{{Cite journal |last1=Harmon |first1=A. D. |last2=Weisgraber |first2=K. H. |last3=Weiss |first3=U. |year=1980 |title=Preformed azulene pigments of ''Lactarius indigo'' (Schw.) Fries (Russulaceae, Basidiomycetes) |journal=Experientia |volume=36 |pages=54–56 |doi=10.1007/BF02003967 |s2cid=21207966}}</ref>
</gallery>
Intense efforts have focused on blue flowers and the possibility that natural blue colourants could be used as food dyes.<ref name=Ag/> Commonly, blue colours in plants are [[Chemistry:Anthocyanin|anthocyanin]]s: "the largest group of water-soluble pigments found widespread in the plant kingdom".<ref>{{Cite book |last=Nuno Mateas, Victor de Freitas |title=Anthocyanins: Biosynthesis, Functions, and Applications |date=2008 |publisher=Springer |isbn=978-0-387-77334-6 |editor-last=Gould |editor-first=K. |page=283 |chapter=Anthrocyanins as Food Colorants |editor-last2=Davies |editor-first2=K. |editor-last3=Winefield |editor-first3=C.}}</ref> In the few plants that exploit structural colouration, brilliant colours are produced by structures within cells. The most brilliant blue colouration known in any living tissue is found in the marble berries of ''[[Biology:Pollia condensata|Pollia condensata]]'', where a spiral structure of cellulose fibrils scattering blue light. The fruit of quandong (''Santalum acuminatum'') can appear blue owing to the same effect.<ref name=Ag/>

===Animals===
<gallery mode="packed">
File:Morpho didius Male Dos MHNT.jpg|Morpho butterfly
File:Indigo Bunting by Dan Pancamo 4.jpg|[[Biology:Indigo bunting|Indigo bunting]]s have iridescent feathers.
File:Mandrill-k-means.png|Blue facial ridges of [[Biology:Mandrill|mandrill]]
File:2009-03-29Dendrobates tinctorius azureus106.jpg|[[Biology:Blue poison dart frog|Blue poison dart frog]]
File:Synchiropus splendidus 2 Luc Viatour cropped.png|The [[Biology:Synchiropus splendidus|mandarin fish]] is one of few animal species with blue pigment
</gallery>
Blue-pigmented animals are relatively rare.<ref>{{Cite journal |last=Umbers |first=Kate D. L. |year=2013 |title=On the Perception, Production and Function of Blue Colouration in Animals |journal=Journal of Zoology |volume=289 |issue=4 |pages=229–242 |doi=10.1111/jzo.12001|bibcode=2013JZoo..289..229U |doi-access=free }}</ref> Examples of which include butterflies of the genus ''[[Biology:Nessaea|Nessaea]]'', where blue is created by [[Chemistry:Pterobilin|pterobilin]].<ref name="Vane-Wright">{{Cite journal |last=Vane-Wright |first=Richard I. |date=22 February 1979 |title=The coloration, identification and phylogeny of ''Nessaea'' butterflies (Lepidoptera: Nymphalidae) |url=https://www.researchgate.net/publication/279190422 |journal=Bulletin of the British Museum (Natural History) |series=Entomology Series |volume=38 |issue=2 |pages=27–56 |access-date=8 February 2018}}</ref> Other blue pigments of animal origin include phorcabilin, used by other butterflies in ''[[Biology:Graphium (butterfly)|Graphium]]'' and ''[[Biology:Papilio|Papilio]]'' (specifically ''[[Biology:Papilio phorcas|P. phorcas]]'' and ''P. weiskei''), and sarpedobilin, which is used by ''[[Biology:Graphium sarpedon|Graphium sarpedon]]''.<ref name="Simonis">{{Cite book |last1=Simonis |first1=Priscilla |title=Photonic Crystals - Introduction, Applications and Theory |last2=Serge |first2=Berthier |date=30 March 2012 |publisher=InTech |isbn=978-953-51-0431-5 |editor-last=Massaro |editor-first=Alessandro |language=English |chapter=Chapter number 1 How Nature produces blue color |access-date=8 February 2018 |chapter-url=https://www.researchgate.net/publication/258042644}}</ref> Blue-pigmented organelles, known as "cyanosomes", exist in the [[Biology:Chromatophore|chromatophore]]s of at least two fish species, the [[Biology:Synchiropus splendidus|mandarin fish]] and the [[Biology:Picturesque dragonet|picturesque dragonet]].<ref>{{Cite journal |author1=Goda, Makoto |author2=Fujii, Ryozo |year=1995 |title=Blue Chromatophores in Two Species of Callionymid Fish |journal=Zoological Science |volume=12 |issue=6 |pages=811–813 |doi=10.2108/zsj.12.811 |s2cid=86385679|doi-access=free }}</ref> More commonly, blueness in animals is a structural colouration; an optical interference effect induced by organised nanometre-sized scales or fibres. Examples include the plumage of several birds like the [[Biology:Blue jay|blue jay]] and [[Biology:Indigo bunting|indigo bunting]],<ref>{{Cite web |date=11 August 2015 |title=How Birds Make Colorful Feathers |url=https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/}}</ref> the scales of butterflies like the morpho butterfly,<ref>{{Cite journal |last1=Potyrailo |first1=Radislav A. |last2=Bonam |first2=Ravi K. |last3=Hartley |first3=John G. |last4=Starkey |first4=Timothy A. |last5=Vukusic |first5=Peter |last6=Vasudev |first6=Milana |last7=Bunning |first7=Timothy |last8=Naik |first8=Rajesh R. |last9=Tang |first9=Zhexiong |last10=Palacios |first10=Manuel A. |last11=Larsen |first11=Michael |year=2015 |title=Towards outperforming conventional sensor arrays with fabricated individual photonic vapour sensors inspired by ''Morpho'' butterflies |journal=Nature Communications |volume=6 |article-number=7959 |bibcode=2015NatCo...6.7959P |doi=10.1038/ncomms8959 |pmc=4569698 |pmid=26324320 |last12=Le Tarte |first12=Laurie A. |last13=Grande |first13=James C. |last14=Zhong |first14=Sheng |last15=Deng |first15=Tao}}</ref> [[Biology:Collagen|collagen]] fibres in the skin of some species of monkey and [[Biology:Opossum|opossum]],<ref name="Prum2004">{{Cite journal |vauthors=Prum RO, Torres RH |date=May 2004 |title=Structural Colouration of Mammalian Skin: Convergent Evolution of Coherently Scattering Dermal Collagen Arrays |url=http://jeb.biologists.org/content/207/12/2157.full.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://jeb.biologists.org/content/207/12/2157.full.pdf |archive-date=9 October 2022 |url-status=live |journal=The Journal of Experimental Biology |volume=207 |issue=Pt 12 |pages=2157–2172 |doi=10.1242/jeb.00989 |pmid=15143148 |bibcode=2004JExpB.207.2157P |hdl=1808/1599 |s2cid=8268610}}</ref> and the [[Biology:Chromatophore#Iridophores and leucophores|iridophore]] cells in some fish and frogs.<ref>{{Cite journal |last1=Ariel Rodríguez |last2=Nicholas I. Mundy |last3=Roberto Ibáñez |last4=Heike Pröhl |year=2020 |title=Being red, blue and green: the genetic basis of coloration differences in the strawberry poison frog (''Oophaga pumilio'') |journal=BMC Genomics |volume=21 |issue=1 |page=301 |doi=10.1186/s12864-020-6719-5 |pmc=7158012 |pmid=32293261 |bibcode=2020BMCG...21..301R |doi-access=free }}</ref><ref>{{Cite journal |last1=Makoto Goda |last2=Ryozo Fujii |year=1998 |title=The Blue Coloration of the Common Surgeonfish, Paracanthurus hepatus—II. Color Revelation and Color Changes |journal=Zoological Science |volume=15 |issue=3 |pages=323–333 |doi=10.2108/zsj.15.323 |pmid=18465994 |s2cid=5860272|doi-access=free }}</ref>

===Eyes===
[[File:A blue eye.jpg|thumb|right|150px|Blue eyes actually contain no blue pigment. The colour is caused by an effect called [[Physics:Tyndall effect|Tyndall scattering]].]]
Blue eyes do not actually contain any blue pigment. Eye colour is determined by two factors: the pigmentation of the eye's [[Medicine:Iris (anatomy)|iris]]<ref name="Wielgus">{{Cite journal |vauthors=Wielgus AR, Sarna T |year=2005 |title=Melanin in human irides of different color and age of donors |journal=Pigment Cell Res. |volume=18 |issue=6 |pages=454–64 |doi=10.1111/j.1600-0749.2005.00268.x |pmid=16280011 |url=https://ruj.uj.edu.pl/xmlui/handle/item/75157 }}</ref><ref name="Prota">{{Cite journal |vauthors=Prota G, Hu DN, Vincensi MR, McCormick SA, Napolitano A |year=1998 |title=Characterization of melanins in human irides and cultured uveal melanocytes from eyes of different colors |journal=Exp. Eye Res. |volume=67 |issue=3 |pages=293–99 |doi=10.1006/exer.1998.0518 |pmid=9778410|doi-access=free }}</ref> and the [[Physics:Scattering|scattering]] of light by the [[Biology:Turbidity|turbid]] medium in the [[Medicine:Stroma of iris|stroma of the iris]].<ref name="Fox">{{Cite book |last=Fox |first=Denis Llewellyn |url=https://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9 |title=Biochromy: Natural Coloration of Living Things |publisher=University of California Press |year=1979 |isbn=978-0-520-03699-4 |page=9 |archive-url=https://web.archive.org/web/20151003232604/https://books.google.com/books?id=c2xyxwlm2UkC&pg=PA9 |archive-date=3 October 2015 |url-status=live}}</ref> In humans, the pigmentation of the iris varies from light brown to black. The appearance of blue, green, and hazel eyes results from the [[Physics:Tyndall effect|Tyndall scattering]] of light in the stroma, an optical effect similar to what accounts for the blueness of the sky.<ref name="Fox" /><ref name="Mason">{{Cite journal |last=Mason |first=Clyde W. |year=1924 |title=Blue Eyes |journal=Journal of Physical Chemistry |volume=28 |issue=5 |pages=498–501 |doi=10.1021/j150239a007 |bibcode=1924JPhCh..28..498M }}</ref> The irises of the eyes of people with blue eyes contain less dark [[Chemistry:Melanin|melanin]] than those of people with brown eyes, which means that they absorb less short-wavelength blue light, which is instead reflected out to the viewer. Eye colour also varies depending on the lighting conditions, especially for lighter-coloured eyes.

Blue eyes are most common in Ireland, the [[Place:Baltic Sea|Baltic Sea]] area and [[Place:Northern Europe|Northern Europe]],<ref name="BostonGlobe" /> and are also found in [[Place:Eastern Europe|Eastern]], [[Place:Central Europe|Central]], and [[Place:Southern Europe|Southern Europe]]. Blue eyes are also found in parts of [[Place:Western Asia|Western Asia]], most notably in Afghanistan, [[Place:Syria|Syria]], Iraq, and [[Religion:Iran|Iran]].<ref name="altervista1">{{Cite web |title=Pigmentation, the Pilous System, and Morphology of the Soft Parts |url=http://carnby.altervista.org/troe/08-05.htm |archive-url=https://web.archive.org/web/20110726095519/http://carnby.altervista.org/troe/08-05.htm |archive-date=26 July 2011 |website=altervista.org}}</ref> In Estonia, 99% of people have blue eyes.<ref>statement by Hans Eiberg from the Department of Cellular and Molecular Medicine at the [[Organization:University of Copenhagen|University of Copenhagen]]</ref><ref name="More than">{{Cite web |last=Weise |first=Elizabeth |date=5 February 2008 |title=More than meets the blue eye: You may all be related |url=https://www.usatoday.com/news/health/2008-02-05-blue-eyes_N.htm |archive-url=https://web.archive.org/web/20120910205153/http://www.usatoday.com/news/health/2008-02-05-blue-eyes_N.htm |archive-date=10 September 2012 |access-date=23 December 2011 |website=USA TODAY}}</ref> In [[Place:Denmark|Denmark]] in 1978, only 8% of the population had brown eyes, though through immigration, today that number is about 11%.<ref name="More than" /> In Germany, about 75% have blue eyes.<ref name="More than" />

In the United States, as of 2006, 1 out of every 6 people, or 16.6% of the total population, and 22.3% of the white population, have blue eyes, compared with about half of Americans born in 1900, and a third of Americans born in 1950. Blue eyes are becoming less common among American children. In the US, males are 3–5% more likely to have blue eyes than females.<ref name="BostonGlobe">{{Cite web |last=Douglas Belkin |date=17 October 2006 |title=Don't it make my blue eyes brown Americans are seeing a dramatic color change |url=https://www.boston.com/yourlife/articles/2006/10/17/dont_it_make_my_blue_eyes_brown/ |url-status=live |archive-url=https://web.archive.org/web/20120223022627/http://www.boston.com/yourlife/articles/2006/10/17/dont_it_make_my_blue_eyes_brown/ |archive-date=23 February 2012 |website=The Boston Globe}}</ref>

== History ==
{{See also|Social:Blue in culture}}

=== In the ancient world ===
<gallery mode="packed" heights="180px">
File:Lapis bowl Iran, AO 26477.jpg|[[Chemistry:Lapis lazuli|Lapis lazuli]] bowl from Iran, end of 3rd – beginning of 2nd millennium BC (Louvre Museum)
File:Tripodic goblet Louvre AO4079.jpg|[[Egyptian blue]] tripodic beaker imitating lapis lazuli. South Mesopotamia. (1399-1200 BC)
File:WLA metmuseum Wall painting Polyphemus and Galaltea 4.jpg|Fresco of Polyphemus and Galatea, Pompei, using [[Egyptian blue]] (1st c. BC) (Metropolitan Museum)
</gallery>
As early as the [[History:7th millennium BC|7th millennium BC]], lapis lazuli was mined in the [[Place:Sar-i Sang|Sar-i Sang]] mines,<ref name="Ashok Roy 2009">David Bomford and Ashok Roy, ''A Closer Look- Colour'' (2009), National Gallery Company, London, ({{ISBN|978-1-85709-442-8}})</ref> in Shortugai, and in other mines in [[Place:Badakhshan|Badakhshan]] province in northeast Afghanistan.<ref>{{Cite book |last=Moorey |first=Peter Roger |url=https://books.google.com/books?id=P_Ixuott4doC&q=Lapis+lazuli+++mines+in+the+Badakhshan&pg=PA86 |title=Ancient Mesopotamian Materials and Industries: the Archaeological Evidence |publisher=Eisenbrauns |year=1999 |isbn=978-1-57506-042-2 |pages=86–87}}</ref>

Lapis lazuli artefacts, dated to 7570 BC, have been found at Bhirrana, which is the oldest site of Indus Valley civilisation.<ref name="lapis">{{Cite web |title=Excavation Bhirrana {{!}} ASI Nagpur |url=http://excnagasi.in/excavation_bhirrana.html |access-date=21 August 2020 |website=excnagasi.in |archive-date=4 August 2020 |archive-url=https://web.archive.org/web/20200804104933/http://excnagasi.in/excavation_bhirrana.html }}</ref> Lapis was highly valued by the Indus Valley Civilisation (7570–1900 BC).<ref name="lapis" /><ref>{{Cite journal |last1=Sarkar |first1=Anindya |last2=Mukherjee |first2=Arati Deshpande |last3=Bera |first3=M. K. |last4=Das |first4=B. |last5=Juyal |first5=Navin |last6=Morthekai |first6=P. |last7=Deshpande |first7=R. D. |last8=Shinde |first8=V. S. |last9=Rao |first9=L. S. |date=25 May 2016 |title=Oxygen isotope in archaeological bioapatites from India: Implications to climate change and decline of Bronze Age Harappan civilization |journal=Scientific Reports |language=en |volume=6 |issue=1 |article-number=26555 |bibcode=2016NatSR...626555S |doi=10.1038/srep26555 |issn=2045-2322 |pmc=4879637 |pmid=27222033 |doi-access=free |s2cid=4425978}}</ref><ref>{{Cite journal |last=DIKSHIT |first=K.N. |date=2012 |title=The Rise of Indian Civilization: Recent Archaeological Evidence from the Plains of 'Lost' River Saraswati and Radio-Metric Dates |journal=Bulletin of the Deccan College Research Institute |volume=72/73 |pages=1–42 |issn=0045-9801 |jstor=43610686}}</ref> Lapis beads have been found at [[History:Neolithic|Neolithic]] burials in Mehrgarh, the [[Earth:Caucasus|Caucasus]], and as far away as Mauritania.<ref name="bc1995">{{Harvcolnb|Bowersox|Chamberlin|1995}}</ref> It was used in the funeral mask of Tutankhamun (1341–1323 BC).<ref>Alessandro Bongioanni & Maria Croce</ref>

A term for Blue was relatively rare in many forms of ancient art and decoration, and even in ancient literature. The Ancient Greek poets described the sea as green, brown or "the colour of wine". The colour is mentioned several times in the [[Religion:Hebrew Bible|Hebrew Bible]] as '[[Tekhelet|tekhelet]]'. Reds, blacks, browns, and ochres are found in cave paintings from the Upper [[Earth:Paleolithic|Paleolithic]] period, but not blue. Blue was also not used for dyeing fabric until long after red, ochre, pink, and purple. This is probably due to the perennial difficulty of making blue dyes and pigments. On the other hand, the rarity of blue pigment made it even more valuable.<ref>See {{harvnb|Pastoureau|2000|pp=13–17}}.</ref>

The earliest known blue dyes were made from plants – woad in Europe, [[Chemistry:Indigo dye|indigo]] in Asia and Africa, while blue pigments were made from minerals, usually either [[Chemistry:Lapis lazuli|lapis lazuli]] or [[Chemistry:Azurite|azurite]], and required more.<ref>{{Cite book |last=Moorey |first=Peter Roger |url=https://books.google.com/books?id=P_Ixuott4doC |title=Ancient mesopotamian materials and industries: the archaeological evidence |publisher=Eisenbrauns |year=1999 |isbn=978-1-57506-042-2 |pages=[https://books.google.com/books?id=P_Ixuott4doC&pg=PA86 86]–[https://books.google.com/books?id=P_Ixuott4doC&pg=PA87 87]}}</ref> Blue glazes posed still another challenge since the early blue dyes and pigments were not thermally robust. In {{Circa|2500 BC}}, the blue glaze [[Egyptian blue]] was introduced for ceramics, as well as many other objects.<ref name="chase">Chase, W.T. 1971, "Egyptian blue as a pigment and ceramic material." In: R. Brill (ed.) ''Science and Archaeology''. Cambridge, Mass: MIT Press. {{ISBN|0-262-02061-0}}</ref><ref>J. Baines, "Color Terminology and Color Classification in Ancient Egyptian Color Terminology and Polychromy", in ''The American Anthropologist'', volume 87, 1985, pp.&nbsp;282–97.</ref> The Greeks imported indigo dye from India, calling it indikon, and they painted with Egyptian blue. Blue was not one of the four primary colours for Greek painting described by Pliny the Elder (red, yellow, black, and white). For the Romans, blue was the colour of mourning, as well as the colour of barbarians. The Celts and Germans reportedly dyed their faces blue to frighten their enemies, and tinted their hair blue when they grew old.<ref>Caesar, ''The Gallic Wars'', V., 14, 2. Cited by Miche Pastourou, p. 178.</ref> The Romans made extensive use of indigo and Egyptian blue pigment, as evidenced, in part, by frescos in Pompeii.
The Romans had many words for varieties of blue, including {{lang|la|caeruleus}}, {{lang|la|caesius}}, {{lang|la|glaucus}}, {{lang|la|cyaneus}}, {{lang|la|lividus}}, {{lang|la|venetus}}, {{lang|la|aerius}}, and {{lang|la|ferreus}}, but two words, both of foreign origin, became the most enduring; {{lang|la|blavus}}, from the Germanic word ''blau'', which eventually became ''bleu'' or blue; and {{lang|la|azureus}}, from the Arabic word {{transliteration|ar|lazaward|engvar=gb}}, which became azure.{{sfn|Pastoureau|2000|p=26}}

Blue was widely used in the decoration of churches in the Byzantine Empire.<ref>L. Brehier, ''Les mosaiques a fond d'azur'', in ''Etudes Byzantines'', volume III, Paris, 1945. pp. 46ff.</ref> By contrast, in the Islamic world, blue was of secondary to green, believed to be the favourite colour of the [[Biography:Muhammad|Prophet Mohammed]]. At certain times in Moorish Spain and other parts of the Islamic world, blue was the colour worn by Christians and Jews, because only Muslims were allowed to wear white and green.{{sfn|Varichon|2005|p=175}}

=== In the Middle Ages ===
<gallery mode="packed" heights="180px">
File:Vitraux Saint-Denis 190110 19.jpg|Stained glass window at Saint Denis Basilica (1130–1140), coloured with [[Chemistry:Cobalt blue|cobalt blue]] 
File:Vitrail Chartres Notre-Dame 210209 1.jpg|Detail of the Blue Virgin Window, Chartres Cathedral (12th c.)
File:Wilton diptych.jpg|The Wilton Diptych (1395–1399). The Virgin Mary was traditionally shown in blue(14th c.)
</gallery>
In the art and life of Europe during the early [[History:Middle Ages|Middle Ages]], blue played a minor role. This changed dramatically between 1130 and 1140 in Paris, when the Abbe Suger rebuilt the Saint Denis Basilica. Suger considered that light was the visible manifestation of the Holy Spirit.<ref>Lours, Mathieu, "Le Vitrail", Editions Jean-Paul Gisserot, Paris (2021)</ref> He installed [[Engineering:Stained glass|stained glass]] windows coloured with [[Chemistry:Cobalt|cobalt]], which, combined with the light from the red glass, filled the church with a bluish violet light. The church became the marvel of the Christian world, and the colour became known as the {{lang|fr|"bleu de Saint-Denis"}}. In the years that followed even more elegant blue stained glass windows were installed in other churches, including at Chartres Cathedral and Sainte-Chapelle in Paris.{{sfn|Pastoureau|2000|pp=44–47}}

In the 12th century the Roman Catholic Church dictated that painters in Italy (and the rest of Europe consequently) to paint the Virgin Mary with blue, which became associated with holiness, humility and virtue. In medieval paintings, blue was used to attract the attention of the viewer to the Virgin Mary.<!-- lapis lazuli, from the mines of Badakshan, in the mountains of Afghanistan, near the source of the Oxus River. The mines were visited by Marco Polo in about 1271; he reported, "here is found a high mountain from which they extract the finest and most beautiful of blues." Ground lapis was used in Byzantine manuscripts as early as the 6th century, but it was impure and varied greatly in colour. Ultramarine refined out the impurities through a long and difficult process, creating a rich and deep blue. It was called {{lang|fr|bleu outremer}} in French and {{lang|it|blu oltremare}} in Italian, since it came from the other side of the sea. It cost far more than any other colour, and it became the luxury colour for the kings and princes of Europe.{{sfn|Ball|2001|p=346}}--> Paintings of the mythical King Arthur began to show him dressed in blue. The coat of arms of the kings of France became an azure or light blue shield, sprinkled with golden [[Fleur-de-lis|fleur-de-lis]] or lilies. Blue had come from obscurity to become the royal colour.{{sfn|Pastoureau|2000|pp=51–52}}

=== Renaissance through 18th century ===

Blue came into wider use beginning in the Renaissance, when artists began to paint the world with perspective, depth, shadows, and light from a single source. In Renaissance paintings, artists tried to create harmonies between blue and red, lightening the blue with lead white paint and adding shadows and highlights. Raphael was a master of this technique, carefully balancing the reds and the blues so no one colour dominated the picture.{{sfn|Ball|2001|p=165}}

[[Ultramarine]] was the most prestigious blue of the Renaissance, being more expensive than gold. Wealthy art patrons commissioned works with the most expensive blues possible. In 1616 Richard Sackville commissioned a portrait of himself by Isaac Oliver with three different blues, including ultramarine pigment for his stockings.<ref>Travis, Time, "The Victoria and Albert Book of Colour Design" (2020), p. 185</ref>

<gallery mode="packed" heights="180">
File:Oliver Richard Sackville Earl of Dorset 1616.jpg|Portrait of Richard Sackville (1616), using three expensive blues, including ultramarine for his stockings
File:MET DP251168.jpg|Ming dynasty, Porcelain vase painted with cobalt blue under transparent glaze. (15th c.) (Metropolitan Museum)
File:Delftware plaque with New Testament scene 002.jpg|Delftware plaque with cobalt blue painting (1683) (Rijksmuseum, Amsterdam)
File:Portrait of Louis XIV of France in Coronation Robes (by Hyacinthe Rigaud) - Louvre Museum.jpg|Portrait of King Louis XIV in coronation robes, by Hyacinthe Rigaud (c. 1700) (Louvre Museum)
File:Urn with cover MET DP104608.jpg|Urn by Josiah Wedgwood (1780s) (Metropolitan Museum)
File:Queen Maria I of Portugal (1734-1816) in an 18th century painting.jpg|Queen Maria I of Portugal (late 1700s)
File:Johannes Vermeer - Girl with a Pearl Earring - WGA24666.jpg|''Girl with a Pearl Earring'' by Johannes Vermeer features [[Ultramarine|ultramarine]] pigment
</gallery>
An industry for the manufacture of fine blue and white pottery began in the 14th century in Jingdezhen, China, using white Chinese porcelain decorated with patterns of [[Chemistry:Cobalt blue|cobalt blue]], imported from Persia. It was first made for the family of the Emperor of China, then was exported around the world, with designs for export adapted to European subjects and tastes. The Chinese blue style was also adapted by Dutch craftsmen in Delft and English craftsmen in Staffordshire in the 17th-18th centuries. in the 18th century, blue and white porcelains were produced by Josiah Wedgwood and other British craftsmen.<ref>Travis, Tim, ''The Victoria and Albert Museum Book of Colour in Design'' (2020), p. 200-201</ref>

===19th-20th century===
<gallery mode="packed" heights="180px">
File:BrummellDighton1805.jpg|Beau Brummell (1776–1840) introduced the ancestor of the modern blue suit
File:D. Maria II (1834) - Joaquim Rafael (Museu Militar de Lisboa).png|Queen Maria II of Portugal in a blue and gold embroidered gown (1835)
File:A Miner in His Cabin.jpg|A California gold miner in blue jeans (1853)
File:Ferdinand Krumholz Isabel do Brasil 1853.jpg|Isabel, Princess Imperial of Brazil in light blue gown (1853)
File:New York Metropolitan Police Uniforms 1871.jpg|New York City police in 1871
</gallery>
The early 19th century saw the ancestor of the modern blue business suit, created by Beau Brummell (1776–1840), who set fashion at the London Court. It also saw the invention of blue jeans, a highly popular form of workers's costume, invented in 1853 by Jacob W. Davis who used metal rivets to strengthen blue denim work clothing in the California gold fields. The invention was funded by San Francisco entrepreneur Levi Strauss, and spread around the world.<ref name="p32" />
<gallery mode="packed" heights="180px">. 
File:Starry Night Over the Rhone.jpg|Van Gogh's ''Starry Night Over the Rhône'' (1888). Blue used to create a mood or atmosphere. A cobalt blue sky, and cobalt or ultramarine water.
File:Matisse Conversation.jpg|''The Conversation'' by Henri Matisse (1908–1912)
</gallery>
Recognising the emotional power of blue, many artists made it the central element of paintings in the 19th and 20th centuries. They included Pablo Picasso, Pavel Kuznetsov and the Blue Rose art group, and Kandinsky and Der Blaue Reiter (The Blue Rider) school.<ref name="autogenerated1">Wassily Kandinsky, M. T. Sadler (Translator) ''Concerning the Spiritual in Art''. Dover Publ. (Paperback). 80 pp.&nbsp;{{ISBN|0-486-23411-8}}.</ref> Henri Matisse expressed deep emotions with blue, "A certain blue penetrates your soul."<ref>{{lang|fr|"Un certain bleu pénètre votre âme."}} Cited in {{harvnb|Riley|1995}}.</ref> In the second half of the 20th century, painters of the abstract expressionist movement use blues to inspire ideas and emotions. Painter [[Biography:Mark Rothko|Mark Rothko]] observed that colour was "only an instrument;" his interest was "in expressing human emotions tragedy, ecstasy, doom, and so on".<ref>''Mark Rothko 1903–1970''. Tate Gallery Publishing, 1987.</ref>

== In society and culture ==
{{Main|Social:Blue in culture}}

=== Uniforms ===
<gallery mode="packed" heights="180">
File:HoratioNelson1.jpg|[[Navy blue]] derives its name from the uniform of Royal Navy officers
File:TSA agents, ca. 2016.jpg|Transportation Security Administration agents
File:2010. Донецк. Карнавал на день города 010.jpg|Ukrainian police officer in Donetsk
File:PM do Rio muda o comando de 25 UPPs.jpg|Officers of the Military Police of Rio de Janeiro State, Brazil
</gallery>
In the 17th century. The Prince-Elector of Brandenburg, Frederick William I of Prussia, chose [[Chemistry:Prussian blue|Prussian blue]] as the new colour of Prussian military uniforms, because it was made with Woad, a local crop, rather than [[Indigo]], which was produced by the colonies of Brandenburg's rival, England. It was worn by the German army until World War I, with the exception of the soldiers of Bavaria, who wore sky-blue.<ref>Heller (2010) p.31</ref>

In 1748, the Royal Navy adopted a dark shade of blue for the uniform of officers.<ref name="p32">Heller (2010) p.32</ref> It was first known as marine blue, now known as [[Navy blue|navy blue]].<ref>J.R. Hill, ''The Oxford Illustrated History of the Royal Navy'', Oxford University Press, 1995.</ref> The militia organised by George Washington selected blue and [[Buff (colour)|buff]], the colours of the British Whig Party. Blue continued to be the colour of the field uniform of the US Army until 1902, and is still the colour of the dress uniform.<ref>{{Cite web |last=Walter H. Bradford |title=Wearing Army Blue: a 200-year Tradition |url=http://www.army.mil/symbols/uniforms/history.html  |archive-url=https://web.archive.org/web/20141119095538/http://www.army.mil/symbols/uniforms/history.html |archive-date=19 November 2014 |website=army.mil}}</ref>

In the 19th century, police in the United Kingdom, including the Metropolitan Police and the City of London Police also adopted a navy blue uniform. Similar traditions were embraced in France and Austria.<ref>Jean Tulard, Jean-François Fayard, Alfred Fierro, ''Histoire et dictionnaire de la Révolution française'', 1789–1799, Éditions Robert Laffont, collection Bouquins, Paris, 1987. {{ISBN|2-7028-2076-X}}</ref> It was also adopted at about the same time for the uniforms of the officers of the New York City Police Department.<ref name="p32" />

=== Gender ===
{{See also|Gendered associations of pink and blue}}
Blue is used to represent [[Biology:Male|male]]s. Beginning as a trend the mid-19th century and applying primarily to clothing, gendered associations with blue became more widespread from the 1950s. The colour became associated with males after [[Social:World War II|World War II]].<ref>{{Cite web |date=2017-12-22 |title=Pink for boys and blue for girls: the colorful history of things designed for kids |url=https://today.usc.edu/pink-for-boys-and-blue-for-girls-the-colorful-history-of-things-designed-for-kids/#:~:text=But%20that%20wasn't%20always,wood%20toys%20over%20plastic%20ones?%E2%80%9D |access-date=2024-10-22 |website=USC Today |language=en-US}}</ref>
<gallery mode="packed" heights="150px">
File:ANAB767300Restroomsign.jpg|This restroom sign on an All Nippon Airways Boeing 767-300 uses pink for the female gender and blue for the male gender.
File:Gender Reveal Cake (28005690402).jpg|Cake using blue to represent the male sex.
File:Fynn Esser 2013-11-18 21-47.jpg|Baby blue newborn male clothing
</gallery>

===Religion===
<gallery mode="packed" heights="180">
File:Sri Mariamman Temple Singapore 2 amk.jpg|In [[Religion:Hinduism|Hinduism]], [[Religion:Krishna|Krishna]] is depicted with blue skin
File:Reknown blue domes of the Church dedicated to St. Spirou in Firostefani, Santorini island (Thira), Greece.jpg|Blue domes of the Church dedicated to St. Spirou in Firostefani, [[Biography:Santorini|Santorini]] island (Thira), Greece.
File:Mezquita Shah, Isfahán, Irán, 2016-09-20, DD 64.jpg|Persian blue in Shah mosque (16th c.) in [[Place:Isfahan|Isfahan]], Iran
</gallery>
* [[Blue in Judaism]]: In the Torah,<ref>Numbers 15:38.</ref> the Israelites were commanded to put fringes, ''tzitzit'', on the corners of their garments, and to weave within these fringes a "twisted thread of blue (''tekhelet'')".<ref>[http://www.tekhelet.com Tekhelet.com] {{webarchive|url=https://web.archive.org/web/20080130063924/http://www.tekhelet.com/ |date=30 January 2008 }}, the Ptil Tekhelet Organization</ref> In ancient days, this blue thread was made from a dye extracted from a Mediterranean snail called the ''hilazon''. [[Biography:Maimonides|Maimonides]] claimed that this blue was the colour of "the clear noonday sky"; Rashi, the colour of the evening sky.<ref>''Mishneh Torah'', ''Tzitzit'' 2:1; Commentary on Numbers 15:38.</ref> According to several rabbinic sages, blue is the colour of God's Glory.<ref>''Numbers Rabbah'' 14:3; ''Hullin'' 89a.</ref> Staring at this colour aids in mediation, bringing us a glimpse of the "pavement of sapphire, like the very sky for purity", which is a likeness of the Throne of God.<ref>Exodus 24:10; Ezekiel 1:26; ''Hullin'' 89a.</ref> (The [[Hebrew language|Hebrew]] word for glory) Many items in the ''Mishkan'', the portable sanctuary in the wilderness, such as the ''menorah'', many of the vessels, and the [[Engineering:Ark of the Covenant|Ark of the Covenant]], were covered with blue cloth when transported from place to place.<ref>Numbers 4:6–12.</ref>
* Blue in [[Religion:Christianity|Christianity]]: Blue is particularly associated with the Virgin Mary. This was the result of a decree of Pope Gregory I (540–601) who ordered that all religious paintings should tell a story which was clearly comprehensible to all viewers, and that figures should be easily recognisable, especially that of the figure of Mary. If she was alone in the image, her costume was usually painted with the finest blue, [[Ultramarine|ultramarine]]. If she was with Christ, her costume was usually painted with a less expensive pigment, to avoid outshining him.<ref name="Heller 2009 p. 32">Heller, "Psychologie de la Colour - Effets et Symboliques", (2009), p. 32</ref><ref>{{Cite web |title=Your question answered |url=http://campus.udayton.edu/mary/questions/faq/faq12.html  |archive-url=https://web.archive.org/web/20060904024808/http://campus.udayton.edu/mary/questions/faq/faq12.html |archive-date=4 September 2006 |website=udayton.edu}}</ref><ref>{{Cite web |title=The Spirit of Notre Dame |url=http://www.nd.edu/~wcawley/corson/schoolcolors.htm |url-status=live |archive-url=https://web.archive.org/web/20111230134831/http://www.nd.edu/~wcawley/corson/schoolcolors.htm |archive-date=30 December 2011 |access-date=31 December 2011 |website=Nd.edu}}</ref><ref>{{Cite web |title=Board Question #31244 &#124; The 100 Hour Board |url=http://theboard.byu.edu/questions/31244/ |url-status=live |archive-url=https://web.archive.org/web/20120331124840/http://theboard.byu.edu/questions/31244/ |archive-date=31 March 2012 |access-date=31 December 2011 |website=Theboard.byu.edu}}</ref>
* Blue in [[Religion:Hinduism|Hinduism]]: Many of the gods are depicted as having blue-coloured skin, particularly those associated with [[Religion:Vishnu|Vishnu]], who is said to be the preserver of the world, and thus intimately connected to water. [[Religion:Krishna|Krishna]] and [[Biology:Rama|Rama]], Vishnu's avatars, are usually depicted with blue skin. [[Religion:Shiva|Shiva]], the destroyer deity, is also depicted in a light-blue hue, and is called ''[[Religion:Nilakanta (Hinduism)|Nīlakaṇṭha]]'', or blue-throated, for having swallowed poison to save the universe during the Samudra Manthana, the churning of the ocean of milk. Blue is used to symbolically represent the fifth, and the throat, [[Philosophy:Chakra|chakra]] ([[Philosophy:Vishuddha|Vishuddha]]).<ref>Stevens, Samantha. ''The Seven Rays: a Universal Guide to the Archangels''. Toronto: Insomniac Press, 2004. {{ISBN|1-894663-49-7}}. p. 24.</ref>
* Blue in [[Religion:Sikhism|Sikhism]]: The Akali Nihangs warriors wear all-blue attire. [[Biography:Guru Gobind Singh|Guru Gobind Singh]] also has a blue roan [[Biology:Horse|horse]]. The Sikh Rehat Maryada states that the [[Religion:Nishan Sahib|Nishan Sahib]] hoisted outside every [[Religion:Gurdwara|Gurudwara]] should be [[Yellow|xanthic]] (Basanti in [[Social:Punjabi language|Punjabi]]) or greyish blue (modern day [[Navy blue|navy blue]]) (Surmaaee in [[Social:Punjabi language|Punjabi]]) colour.<ref>Sikh Rehat Maryada: [https://old.sgpc.net/rehat_maryada/section_three_chap_four.htm Section Three, Chapter IV, Article V, r.]</ref><ref>{{Cite web|url=https://www.sikhmuseum.com/nishan/|title=Nishan Sahib Khanda Sikh Symbols Sikh Museum History Heritage Sikhs|website=www.sikhmuseum.com}}</ref>

=== Sports ===
<gallery mode="packed" heights="180">
File:Italy Team - Rome, 1965.jpg|The Italian national football team
File:Serbia national volleyball team at the 2012 Summer Olympics (7913882066).jpg|Serbian national volleyball team, 2012 Olympics
File:Jack White, Duke Blue Devils.jpg|Basketball player for the Blue Devils at Cassell Coliseum
</gallery>
In sports, blue is widely represented in uniforms in part because the majority of national teams wear the colours of their national flag. For example, the national men's football team of France are known as ''Les Bleus'' (the Blues). Similarly, Argentina, Italy, and Uruguay wear blue shirts.<ref>{{Cite web |title=FIFA World Cup 2010 – Historical Football Kits |url=http://historicalkits.co.uk/international/tournaments/fifa_world_cup_2010/fifa-world-cup-2010.html |url-status=live |archive-url=https://web.archive.org/web/20120107043904/http://www.historicalkits.co.uk/international/tournaments/fifa_world_cup_2010/fifa-world-cup-2010.html |archive-date=7 January 2012 |access-date=31 December 2011 |website=Historicalkits.co.uk}}</ref> The Asian Football Confederation and the Oceania Football Confederation use blue text on their logos. Blue is well represented in baseball (Blue Jays), basketball, and American football, and ice hockey. The Indian national cricket team wears blue uniform during One day international matches, as such the team is also referred to as "Men in Blue".<ref>{{Cite news |date=3 July 2016 |title=This Is The Reason Why Indian Cricket Team Wears A Blue Jersey During ODIs |url=https://www.indiatimes.com/sports/this-is-the-reason-why-indian-cricket-team-wears-a-blue-jersey-during-odis-257727.html}}</ref>

===Politics===
<gallery mode="packed" heights="150">
File:Flag of the United Nations.svg|Flag of the United Nations, approximates "sky blue"
File:Flag of Europe.svg|Flag of the [[Social:European Union|European Union]] is "reflex blue", a medium dark blue
File:Red states and blue states of the US based on data from the 2008, 2012, 2016 and 2020 presidential elections.svg|A presidential-election map of the US, 2008&ndash;2020. States that consistently vote for Democrats are termed "blue states".
</gallery>
Unlike red or green, blue was not strongly associated with any particular country, religion or political movement. As the colour of harmony, it was chosen as the colour for the flags of the United Nations, the [[Social:European Union|European Union]], and NATO.<ref name="PsyCoul">Heller, "Psychologie de la Couleur" pp. 36-37</ref> In politics, blue is often used as the colour of conservative parties, contrasting with the red associated with left-wing parties.<ref>{{Cite book |last=Stone |first=Terry Lee |url=http://archive.org/details/colordesignworkb0000ston |title=Color design workbook: a real-world guide to using color in graphic design |date=2006 |publisher=Gloucester, Mass. : Rockport Publishers |others=Internet Archive |isbn=978-1-59253-192-9}}</ref> Some conservative parties that use the colour blue include the [[Finance:Conservative Party (UK)|Conservative Party (UK)]],<ref>{{Cite web |date=20 April 2006 |title=Why is the Conservative Party blue? |url=https://news.bbc.co.uk/1/hi/magazine/4923050.stm |access-date=23 April 2018 |website=BBC News}}</ref> [[Finance:Conservative Party of Canada|Conservative Party of Canada]],<ref>{{Cite web |title=Official Logos |url=https://www.conservative.ca/official-logos/ |access-date=13 February 2024 |website=Conservative Party of Canada |language=en-US}}</ref> Liberal Party of Australia,<ref>{{Cite web |title=What are the colours of the Australian political parties? - Parliamentary Education Office |url=https://peo.gov.au/understand-our-parliament/your-questions-on-notice |access-date=13 February 2024 |website=peo.gov.au |language=en}}</ref> Liberal Party of Brazil, and Likud of Israel. However, in some countries, blue is not associated main conservative party. In the United States, the liberal Democratic Party is associated with blue, while the conservative Republican Party is associated with red. US states which have been won by the Democratic Party in four consecutive presidential elections are termed "blue states", while those that have been won by the Republican Party are termed "red states".<ref name="latmpexp16">{{Cite news |last=Battaglio |first=Stephen |date=3 November 2016 |title=When red meant Democratic and blue was Republican. A brief history of TV electoral maps |work=Los Angeles Times |url=https://www.latimes.com/entertainment/tv/la-et-st-electoral-map-20161102-htmlstory.html |access-date=28 November 2018}}</ref> [[Earth:South Korea|South Korea]] also uses this colour model, with the Democratic Party on the left using blue<ref>{{Cite web |date=8 March 2023 |title=더불어민주당 |url=https://theminjoo.kr/introduce/logo |access-date=13 February 2024 |archive-url=https://web.archive.org/web/20230308211933/https://theminjoo.kr/introduce/logo |archive-date=8 March 2023 }}</ref> and the People Power Party on the right using red.

==See also==
* [[Chemistry:Engineer's blue|Engineer's blue]]
* Lists of colours
* [[Non-photo blue]]
* Blue pigments
{{srt}}

==References==
{{reflist}}

==Works cited==
* {{Cite book |last=Ball |first=Philip |title=Bright Earth, Art, and the Invention of Colour |publisher=Penguin Group |year=2001 |isbn=978-2-7541-0503-3 |location=London |page=507}} (page numbers refer to the French translation)
* {{Cite book |last1=Bowersox |first1=Gary W. |last2=Chamberlin |first2=Bonita E. |year=1995 |title=Gemstones of Afghanistan |location=Tucson, AZ |publisher=Geoscience Press}}
* {{Cite book |last=Heller |first=Eva |title=Psychologie de la couleur: effets et symboliques |publisher=Pyramyd |year=2009 |isbn=978-2-35017-156-2 |location=Munich |language=fr}}
* {{Cite book |last=Pastoureau |first=Michel |title=Bleu: Histoire d'une couleur |publisher=Editions du Seuil |year=2000 |isbn=978-2-02-086991-1 |location=Paris |language=fr}}
* {{cite book |last=Riley |first=Charles A. II |year=1995 |title=Color Codes: Modern Theories of Color in Philosophy, Painting and Architecture, Literature, Music, and Psychology |location=Hanover, New Hampshire |publisher=University Press of New England}}
* {{Cite book |last=Travis |first=Tim |title=The Victoria and Albert Museum Book of Colour in Design |publisher=Thames & Hudson |year=2020 |isbn=978-0-500-48027-4}}
* {{Cite book |last=Varichon |first=Anne |title=Couleurs&nbsp;: pigments et teintures dans les mains des peuples |publisher=Editions du Seuil |year=2005 |isbn=978-2-02-084697-4 |location=Paris |language=fr}}
* {{Cite book |last=Lours |first=Mathieu |title=Le Vitrai |publisher=Éditions Jean-Paul Gisserot |year=2020 |isbn=978-2-755-80845-2}}

==Further reading==
* {{Cite book |last=Balfour-Paul |first=Jenny |title=Indigo |publisher=British Museum Press |year=1998 |isbn=978-0-7141-1776-8 |location=London}}
* {{Cite journal |last1=Josserand |first1=M. |last2=Meeussen |first2=E. |last3=Majid |first3=A. |date=27 September 2021 |title=Environment and culture shape both the colour lexicon and the genetics of colour perception |journal=Sci Rep |language=en |publisher=Nature |volume=11 |issue=19095 |page=19095 |doi=10.1038/s41598-021-98550-3 |pmid=34580373 |pmc=8476573 |bibcode=2021NatSR..1119095J |s2cid=238202924}}
* {{Cite web |last=Macdonald |first=Fiona |date=7 April 2018 |title=There's Evidence Humans Didn't Actually See Blue Until Modern Times |url=https://www.sciencealert.com/humans-didn-t-see-the-colour-blue-until-modern-times-evidence-science |access-date=24 June 2022 |website=Science Alert |language=en |archive-date=24 June 2022 |archive-url=https://web.archive.org/web/20220624002709/https://www.sciencealert.com/humans-didn-t-see-the-colour-blue-until-modern-times-evidence-science }}
* {{Cite book |last=Mollo |first=John |title=Uniforms of The American Revolution in Color |publisher=Stirling Publications |others=Illustrated by Malcolm McGregor |year=1991 |isbn=978-0-8069-8240-3 |location=New York}}

==External links==
* [http://theconversation.com/friday-essay-from-the-great-wave-to-starry-night-how-a-blue-pigment-changed-the-world-81031 "Friday essay: from the Great Wave to Starry Night, how a blue pigment changed the world", By Hugh Davies, theconversation.com]

{{Shades of blue}}
{{Shades of light blue}}
{{Shades of violet}}
{{Shades of lavender}}
{{Shades of cyan}}
{{Shades of indigo}}
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[[Category:Optical spectrum]]
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{{Sourceattribution|Blue}}