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 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. Blue is the complement of yellow.
The English language commonly uses "blue" to refer to any colour from blue to cyan.
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 People's Republic of China changed the colour 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.
Blue is used to denote the working class (due to the usual colour 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.
"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. Participants in sporting fixtures between these universities are said to have been awarded "a blue", similar in concept to a varsity letter.
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.
Blue is associated with manhood, as pink is associated with womanhood.
Nationally
In Australia, a "blue" can also describe a fight or an argument. Men with red hair may be nicknamed "Bluey".
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 colour of Italy.
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.
Blue is a pop group. Its members are Duncan James, Simon Webbe, Antony Costa and Lee Ryan. They have released three albums: All Rise, One Love, and Guilty.
Traditionally, blue has been considered as a primary color in painting, with the secondary colour orange as its complement, but this is not consistent with modern scientific color theory. As the mixing of pigments is a subtractive colour process, the true primary colors in painting and printing are cyan, magenta and yellow (with black often added for practical reasons).
