blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal mixture of red and 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 (based on the Munsell colour wheel).
in Modern English, «blue» is one of the basic colour terms, and one of the seven spectral colours, intermediate between violet (purple) and cyan. It comprises a considerable number of identifiable subcategories that can be identified with descriptive terms like navy blue (a dark blue), cyan blue (or «blue-green», on the boundary to the green range), or sky blue (azure).
the word itself was loaned into Middle English from the Old French word bleu, blo » «pale, pallid, discoloured; blue, blue-gray», itself from an Old Frankish *blao.
Etymology and definitions
The modern English word blue comes from Middle English bleu or blewe, from Old French bleu, bleve, blöe, a word of Germanic origin (Frankish or possibly Old High German blāo, «pale, wan, blue-grey»).
Bleu replaced Old English blāw «blue» and blǣwen «light blue». The root of all these variations is Proto-Germanic blǣwaz, from Proto-Indo-European *bhlāw-, *bhlēw- «light-coloured, yellow, grey, blue», from *bhel- «to shine, be light or bright», also the root of Old Norse blār and the modern Icelandic blár, and the Scandinavian word blå, which can also refer to other non blue colours. Also in the French, bleau is a contraction of belleaue or belle eau, meaning «beautiful water» (see Fontainebleau). In this vein, «blue» is the colour of beautiful water as it clearly reflects the blue sky. A Scots and Scottish English word for «blue-grey» is blae, from the Middle English bla («dark blue», from Old Norse blār). Also related is the English word blee meaning «colour, complexion». Ancient Greek lacked a word for blue and Homer called the colour of the sea «wine dark», except that the word kyanos (cyan) was used for dark blue enamel.
The word blue is thought to be cognate with black, blond, originally designating a discoloured, pale, washed-out shade. Through a Proto-Indo-European root, it is also linked with Latin flavus («yellow»; see flavescent and flavine), with Greek phalos (white), French blanc (white, blank) (borrowed from Old Frankish), and with Russian белый, belyi («white,» see beluga), and Welsh blawr (grey) all of which derive (according to the American Heritage Dictionary) from the Proto-Indo-European root *bhel- meaning «to shine, flash or burn», (more specifically the word bhle-was, which meant light coloured, blue, blond, or yellow), whence came the names of various bright colours, and that of colour black from a derivation meaning «burnt» (other words derived from the root *bhel- include bleach, bleak, blind, blink, blank, blush, blaze, flame, fulminate, flagrant and phlegm).
In the English language, blue may refer to the feeling of sadness. «He was feeling blue». This is because blue was related to rain, or storms, and in Greek mythology, the god Zeus would make rain when he was sad (crying), and a storm when he was angry. Kyanos was a name used in Ancient Greek to refer to dark blue tile (in English it means blue-green or cyan).
Many languages do not have separate terms for blue and or green, instead using a cover term for both (when the issue is discussed in linguistics, this cover term is sometimes called grue in English).
Pigments and dyes
Traditionally, blue has been considered a primary colour in painting, with the secondary colour orange as its complement.
Blue pigments include azurite (Cu3(CO3)2(OH)2), ultramarine (Na8-10Al6Si6O24S2-4), cerulean blue (primarily cobalt (II) stanate: Co2SnO4), cobalt blue (cobalt(II) aluminate: CoAl2O4), and Prussian blue (milori blue: primarily Fe7(CN)18).
Traditionally natural dyes such as woad and true indigo were used to produce indigo dye used to colour fabrics blue or indigo. These have now largely been replaced by synthetic dyes.
Scientific natural standards for blue
- Emission spectrum of Cu2+
- Electronic spectrum of aqua-ions Cu(H2O)
- When an animal’s coat is described as «blue», it usually refers to a shade of grey that takes on a bluish tint, a diluted variant of a pure black coat. This designation is used for a variety of animals, including dog coats, some rat coats, cat coats, some chicken breeds, some horse coat colours and rabbit coat colours. Some animals, such as giraffes and lizards, also have blue tongues.
- Lasers emitting in the blue region of the spectrum became widely available to the public in 2010 with the release of inexpensive high-powered 445-447nm Laser diode technology.  Previously the blue wavelengths were accessible only through DPSS which are comparatively expensive and inefficient, however these technologies are still widely used by the scientific community for applications including Optogenetics, Raman spectroscopy, and Particle image velocimetry, due to their superior beam quality. Blue Gas lasers are also still commonly used for Holography, DNA sequencing, Optical pumping, and other scientific and medical applications.
- In the English language, blue often represents the human emotion of sadness, for example, «He was feeling blue». In German, on the other hand, to be «blue» (blau sein) is to be drunk. This derives from the ancient use of urine (which is produced copiously by the human body after drinking alcohol) in dyeing cloth blue with woad or indigo. It may also be in relation to rain, which is usually regarded as a trigger of depressive emotions.
- Conversely blue, a very popular colour can represent happiness and optimism as days with clearer, blue skies tend to be considered times where these emotions are more easily expressed. Many artistic contributions have been made referencing clear days with blue skies as part of the happiness or as a symbolism of the happiness the artist felt, such as Tony Bennett’s Put on a Happy Face. If this were untrue there would obviously be more complaints about days with clear blue skies.
- Blue is commonly used in the Western hemisphere to symbolize the male gender in contrast to pink used for females, although in the early 1900s, blue was the colour for girls (as it had traditionally been the colour of the Virgin Mary in Western Art) and pink was for boys (as it was akin to the colour red, considered a masculine colour.
- Azzurro, a light blue, is the national colour of Italy (from the livery colour of the former reigning family, the House of Savoy).
- Blue is the national sports colour for India, as it denotes secularism.
- Blue is the national colour used on flags of several countries surrounded by seas or oceans such as Australia and Europe, though not necessarily with this interpretation in mind.
- Blue and white are the national colours of Scotland, Argentina, El Salvador, Finland, Greece, Guatemala, Honduras, Israel, Micronesia, Nicaragua, and Somalia as well as of the United Nations.
- Blue, white and yellow are the national colours of Bosnia and Herzegovina, Kosovo and Uruguay.
- Blue, white and black are the national colours of Estonia.
- Blue and yellow are the national colours of Barbados, Kazakhstan, Palau, Sweden, and Ukraine.
- Blue, yellow and green are the national colours of Brazil, Gabon, and Rwanda.
- Blue, yellow and red are the national colours of Chad, Colombia, Ecuador, Moldova, Romania, and Venezuela.
- Blue and red are the national colours of Haiti and Liechtenstein.
- Blue, red and white are the national colours of Cambodia, Costa Rica, Chile, Croatia, Cuba, the Czech Republic, the Dominican Republic, France, Iceland, North Korea, Laos, Liberia, Luxembourg, Nepal, the Netherlands, New Zealand, Norway, Panama, Paraguay, Puerto Rico, Russia, Samoa, Serbia, Slovakia, Slovenia, Thailand, the United Kingdom, and the United States.
- In Hinduism, blue is used to symbolically represent the fifth, throat chakra (Vishuddha).
- The Blue House is the residence of the President of South Korea.
- Blue has been associated with a variety of political positions, often differentiated from communist red or anarchist black. During the revolt in the Vendée against the French Revolution, blues stood for the revolutionary forces, and white for the counter-revolutionaries. Later movements like the Breton blues used the colour to signify allegiance to the ideals of the revolution.
- The blueshirts was a quasi-fascist political organisation active in Ireland during the 1930s (the name comes from the fact that St. Patrick’s Blue is one of the traditional colours of Ireland).
- Blue is the colour of the Conservative Party in Britain and Conservative Party of Canada. Television coverage in the United States since the 2000 presidential election has made it fashionable to link the Democratic Party to «blue» and the Republican Party to «red» (especially in reference to «red states and blue states»). In Brazil, blue states are the ones in which the Social Democratic Party has the majority, in opposition to the Workers’ Party, usually represented by red.
- A blue law is a type of law, typically found in the United States and Canada, designed to enforce religious standards, particularly the observance of Sunday as a day of worship or rest, and a restriction on Sunday shopping. The word blue was used in the 17th century as a disparaging reference to rigid moral codes and those who observed them, particularly in blue-stocking, a reference to Oliver Cromwell’s supporters in the parliament of 1653.
- Blue is associated with numerous centre-right liberal political parties in Europe, Including the People’s Party for Freedom and Democracy (Netherlands), the Reformist Movement and Open VLD (Belgium), the Democratic Party (Luxembourg), Liberal Party (Denmark) and Liberal People’s Party (Sweden).
- Blue is the colour of, and associated with, the New Progressive Party of Puerto Rico.
- Blue is associated in Christianity generally and Catholicism in particular, with the Virgin Mary.
- Blue in Hinduism: Many of the gods are depicted as having blue-coloured skin, particularly those associated with Vishnu, who is said to be the Preserver of the world and thus intimately connected to water. Krishna and Ram, Vishnu’s avatars, are usually blue. Shiva, the Destroyer, is also depicted in light blue tones and is called neela kantha, or blue-throated, for having swallowed poison in an attempt to turn the tide of a battle between the gods and demons in the gods’ favour.
- Blue in Judaism: In the Torah, 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)». In ancient days, this blue thread was made from a dye extracted from a Mediterranean snail called the hilazon. Maimonides claimed that this blue was the colour of «the clear noonday sky»; Rashi, the colour of the evening sky. According to several rabbinic sages, blue is the colour of God’s Glory. 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. (The 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 Ark of the Covenant, were covered with blue cloth when transported from place to place.
- Blue in Islam: In verse 20:102 of the Qur’an, the word زرق zurq (plural of azraq ‘blue’). Blue amulets made of Lapis lazuli are commonly utilized to symbolize luck in some Muslim cultures.
- In Thailand, blue is associated with Friday on the Thai solar calendar. Anyone may wear blue on Fridays and anyone born on a Friday may adopt blue as their colour. The Thai language, however, is one that has had trouble distinguishing blue from green. The default word for Blue was recently สีน้ำเงิน literally, the colour of silver, a poetical reference to the silvery sheen of the deep blue sea. It now means navy blue, and the default word is now สีฟ้า literally, the colour of the sky.