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Blue (from Old High German "blao" shining) is one of the three primary additive colors; blue light has the shortest wavelength (about 470 nm) of the three primary colors.

A clear sky on a sunny day is colored blue because of Rayleigh scattering of the light from the Sun. Large amounts of water ( H₂ O) look blue because red light around 750 nm is absorbed as an overtone of the O-H stretching vibration. Interestingly, heavy water ( D₂ O) is colorless, because the absorption band is at a longer wavelength (~950 nm).

An example of a blue color in the RGB color space has intensities [0, 0, 255] on a 0 to 255 scale. Blue is the complement of yellowYellow is the color of light whose wavelength is between 565nm and 590nm, or is a mixture of red and green light that appears to be the same color. Yellow is one of the subtractive primary colours ( primary pigments), and its complementary color is blue..

The English language commonly uses "blue" to refer to any color from blue to cyanThis article is about the color. For other uses see Cyan (disambiguation Cyan is a pure spectral color, but the same hue can also be generated by mixing equal amounts of green and blue light. As such, cyan is the complement of red: cyan pigments absorb re. Many languages do not have separate terms for blue and greenGreen is a colour seen commonly in nature. Plants are green because they contain chlorophyll. Green light has a wavelength of around 550 nm and is one of the additive primary colours, the complement of magenta. Many artists, however, continue to use a tra.

1 Usage, symbolism, colloquial expressions

• A "blue joke" or "blue comedy" is comedy which uses references to socially tabooFor the Taboo boardgame, see Taboo (game). A taboo is a strong social prohibition (or ban) relating to any area of human activity or social custom declared as sacred and forbidden; breaking of the taboo is usually considered objectionable or abhorrent by subjects such as sexual or lavatorial double entendreA Double entendre or Innuendo is a figure of speech similar to the pun, in which a spoken phrase can be understood in either of two ways. The first, literal meaning is an innocent one, while the second meaning is usually indecently sexual. Although a Fren.
• The colloquial expression "blue" is used to describe melancholy or sadness in English speaking countries. See also BluesBlues is a vocal and instrumental musical form which evolved from African American spirituals, shouts, work songs and chants and has its earliest stylistic roots in West Africa. Blues has been a major influence on later American and Western popular music, music. But "blue skies," referring to sunny weather, implies cheerfulness.
• Vocations:
  • "Blue" often refers to the police force, e.g. "Boys in Blue," "the blue line," or "the blue wall"; it is the color of many police uniforms. Police in the People's Republic of China changed the color of the uniforms from green to blue in the late 1990s partly to emphasize their civilian role.
  • Blue is associated with many air forces and navies from the color of their dress uniforms, navy blue for the latter. The Blue Angels are an acrobatic flight squadron of the U.S. Navy.
  • Blue may denote the working class, derived from the traditional color of factory or shop uniforms. Blue-collar workers are industrial workers as opposed to white-collar office workers.
• In contrast to "blue collar," the phrase "of blue blood" is used to mean "from an aristocratic background," because a pale, untanned skin allows blue-tinged veins to show through.
• Blue is traditionally associated with boys, as pink is associated with girls. "Baby blue" is a pastel color associated with clothing or linens for baby boys.
• Politically:
  • Internationally, blue is the color for conservatives (with notable exceptions; see Political party).
  • In the United States since the 2000 presidential election, " blue states" (or counties, or people) are invariably those favoring the Democratic Party, and blue is by extension the Democratic color. Previously, electoral maps either used blue to represent the incumbent and red for the challenger, or alternated. Separately, conservative Southern Democrats were nicknamed "blue dogs" (see below).
  • Those in the United Kingdom and Canada use blue for the Conservative Party and Conservative Party of Canada, respectively (which use blue as one of their symbols).
  • In Germany, blue and yellow are the colors of the liberal party ( FDP).
  • In the Republic of China on Taiwan, the blue is the symbol for the Kuomintang and has been used as the emblem for Chinese reunification leaning parties.
  • Blue, like white may represent authority as opposed to revolutionary red or black.
• A "blue book" is an almanac or similar reference work. For instance, the Oregon Blue Book is the official directory and fact repository of the state of Oregon. The Harvard Bluebook dictates a style of legal citation. The Kelley Blue Book is a popular guide to used car prices.
• "Blue laws" is a slang term for laws regulating issues of morality, such as alcohol, gambling, or sexually-explicit materials.
• The Blue Riband is a prize awarded since the 1860s to the ship that made the fastest transatlantic crossing.
• " Blue ribbon" is a term used to describe something of high quality, such as a blue ribbon panel or a blue ribbon commission. This usage comes from the practice of awarding blue ribbons for first place in certain athletic or other competitive endeavours.
• Dark blue is associated with Oxford University and light blue with Cambridge University. The Sporting Colours of these universities are called blues.
• Blue is often denoted injury since it is the color of a bruise.
• In medical diagrams, blue is used to represent veins carrying deoxygenated blood back to the heart. Deoxygenated blood is actually reddish violet.
• Users of Microsoft Windows often use the term "blue" to describe a computer that has encountered a " blue screen of death."
• Nationally
  • In Australia, a "blue" can also describe a fight or an argument. Men with red hair may be nicknamed "Bluey".
  • The German word for blue is used for "drunk".
  • In Russian, the word for light blue is slang for " gay".
  • Blue movie is a slang term for a pornographic film. There are also "blue magazines". This term is most common in Britain but also used in the United States and Israel.
  • Azzurro (a light blue) is the national color of Italy.
• Blue is used to represent the Union in the American Civil War in opposition to the Confederate gray, both based on the uniforms worn by the respective armies.
• "Big Blue" is a nickname for IBM. Deep Blue was its chess-playing computer which defeated Garry Kasparov.
• A blue moon is the second full moon in a calendar month, the third full moon in a season that has four, or a moon that appears blue because of particles in the atmosphere. All are uncommon enough that "once in a blue moon" means "almost never."
• A blue note is a note between the regular notes on the scale. Blue notes are the most important notes in the blues scale.
• A blue dog has a coat color that is primarily gray or silver. For example, see Kerry Blue Terrier for a solid "blue" coat or Australian Shepherd for blue merle .
• Blue pages are a telephone directory of government offices, either an official "blue book" or a section of a commercial directory, contrasted with the yellow pages or white pages.
• In The Hitchhiker's Guide to the Galaxy there are several references to the Hooloovoo, "a super-intelligent shade of the colour blue".