White is the sum of all light wavelengths in equal proportions. In additive color it is the result of mixing red, green, and blue at full intensity, while in subtractive color it is the absence of all color, allowing the white of the paper to come through unaltered.

White

White isn't quite as absolute to color scientists as it might be to the lay person. The CIE (Commission Internationale de L'Eclairage, or International Commission on Illumination) has several standard illuminants, such as incandescent (known as illuminant A), solar (D65), and fluorescent (F2) sources. Each of these illuminants has a different wavelength profile which causes colors to look differently. This is why sometimes clothes tried on under bright store dressing room lights don't look the same at home. Any of these illuminants can be said to produce "white" light.

CIE standard illuminants correspond roughly to blackbody radiation temperatures (sometimes called color temperatures). Blackbody radiation is light emitted from a blackbody, an object that does not reflect any light; it is strictly a light producer. Examples of blackbodies are the sun, a candle flame, or the flame of a welding torch. Note that these are all hot objects that emit light. Each of these blackbodies is a different temperature, and thus emits different spectra of light. In opposition to common sense, the hottest blackbodies emit blue light, while relatively cooler blackbodies appear redder. Blackbody temperatures are measured in Kelvins (the same scale as celsius proportionately, but 0 Kelvins = −273 degrees celsius). The standard illuminant E is equivalent to 5400 Kelvin and is the point of equal energy output from all wavelengths.

Simulation of various color temperatures. Since your monitor has a color temperature all its own, these are only approximated colors.

On the psychological side, white represents purity, chastity, cleanliness, refreshment, and innocence. Because it reflects light, it stays cooler and is often worn in summer. It is chromatically neutral and is thus a good background for viewing things critically, such as artwork and photographs. However, if the background is too bright it can cause glare and strain the eyes. It can be difficult to distinguish two similar dark shades against a white background.