The Online Encyclopedia and Dictionary







Blue (from Old High German "blao" shining) is one of the three primary additive colors; blue light has the shortest wavelength range (about 420-490 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 (H2O) look blue because red light around 750 nm is absorbed as an overtone of the O-H stretching vibration. Interestingly, heavy water (D2O) is colorless, because the absorption band (~950 nm) is outside the visible spectrum.

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 yellow. For this reason, blue 80A filters are used to correct for the excessive redness of tungsten lighting in color photography.

The English language commonly uses "blue" to refer to any color from blue to cyan. Many languages do not have separate terms for blue and green, and in the Swedish language, blue was also used for black until the early 20th century.


Usage, symbolism, colloquial expressions

  • A "blue joke" or "blue comedy" is comedy which uses references to socially taboo subjects such as sexual or lavatorial double entendre.
  • The colloquial expression "blue" is used to describe melancholy or sadness in English speaking countries. See also Blues 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 nowadays associated with boys, and pink is associated with girls. Before the 20th century, it was vice versa.
  • Politically:
  • 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. The Blue Book refers to a policy document issued by the Federal Communications Commission in 1946, urging television networks to uphold their commitment to public service.
  • "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.
  • Blue is the name of a credit card with a smart-chip issued by American Express.
  • In The Hitchhiker's Guide to the Galaxy there are several references to the Hooloovoo, "a super-intelligent shade of the colour blue".
  • On Star Trek, medical and scientific personnel wear blue uniforms.
  • Blue is the colour of the snooker ball which has a 5-point value.
House painted blue
House painted blue


  • Blues (music genre)
  • Blue (rock band) are a former British rock band.
  • Blue are a British boy band. Its members are Duncan James, Simon Webbe, Antony Costa and Lee Ryan. They have released three albums: All Rise, One Love, and Guilty.
  • Blue is the title of an album by the Canadian singer/songwriter Joni Mitchell.
  • Kind of Blue is the title of an album by Miles Davis, which has become one of the biggest selling jazz recordings in history.
  • Blue Train is the title of an influential jazz album by John Coltrane
  • Rhapsody in Blue is a symphonic jazz composition for jazz band, piano, and orchestra by George Gershwin.
  • Love is Blue is a popular tune from the 1960's by Andy Williams, most notably performed by Paul Mauriat.
  • Blue has been used as a song title by many artists, notably LeAnn Rimes and Eiffel 65.

Use in painting

Traditionally, blue has been considered as a primary color in painting, with the secondary color orange as its complement, but this is not consistent with modern scientific color theory. As the mixing of pigments is a subtractive color process, the true primary colors in painting and printing are cyan, magenta and yellow (with black often added for practical reasons; see CMYK color model).

Use in Statistics

BLUE is an acronym for the Best Linear Unbiased Estimator .

See also

External links

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