Blue

Blue is one of the three primary additive colors; blue light has the shortest wavelength (about 470 nm) of the three primary colours.

A clear sky on a sunny day is coloured 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 colourless, 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. On a browser that supports visual formatting in Cascading Style Sheets, the following box should appear in this color:

The English language commonly uses "blue" to refer to any color from blue to cyan.

Usage, symbolism, colloquial expressions

The colloquial expression "blue" is used to describe melancholy or sadness in English speaking countries. See also Blues music.
The word "blue" is often used in reference to the Police force; e.g. "Boys in Blue", "the blue line"
Blue is also the color of many police uniforms. Police in the Peoples 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 the color of many air force dress uniforms including those of the United States.
Blue is used to denote the working class (due to the usual color of working clothes). Blue-collar workers are industrial workers as opposed to white-collar office workers.
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.
A "Blue movie" is a slang term for a pornographic film. There are also "Blue magazines". This term is more common in Great Britain than the United States.
Users of Microsoft Windows often use the term "blue" to describe a computer that has encountered a blue screen of death.
"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 endeavors.
Dark blue is associated with Oxford University and light blue with Cambridge University. Participants in sporting fixtures between these universities are said to have been awarded "a blue", similar in concept to a varsity letter.
In Australia, a "blue" can also describe a fight or an argument.
The German word for blue is used for "drunk".
In Japanese, the word for blue is used for "young".
In Russian, the word for blue is slang for "gay".
Blue 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.
Politically: Electoral maps in the United States usually use blue for the Democratic Party. Those in the United Kingdom use blue for the Conservative Party (which uses blue as one of its symbols), and those in Canada use blue to refer to the Progressive Conservatives. 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. Internationally, blue is the color for conservatives (with notable exceptions; see Political party).
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.
Azzurro (a light blue) is the national color of Italy.
"Big Blue" is a nickname for IBM.
A blue moon is the second full moon in a calendar month, or a moon that appears blue because of particles in the atmosphere. Both 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.

House painted blue

Music

Blue is also 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 is also a title for songs from LeAnn Rimes to Eiffel 65.