Hue

A hue refers to the gradation of color within the optical spectrum, or visible spectrum, of light. "Hue" may also refer to a particular color within this spectrum, as defined by its dominant wavelength, or the central tendency of its combined wavelengths. For example, a light wave with a central tendency within 565-590 nm will be yellow.

In painting color theory, a hue refers to a pure color &mdash;one without added white (tint) or black (shade) pigment.

In an RGB color space, hue can be thought of as an angle &phi; in standard position. To calculate &phi;, let R, G, B be the color coordinates in RGB space, defined on a scale from zero to one. Then, after obtaining the brightness &mu; and the saturation &sigma;, the hue could be obtained from
 * $$ \phi = \arccos \left( {R - \mu \over \sigma \sqrt{2}} \right) $$

(Compare with standard score). Using this formula, &phi;=0 (in radians) would correspond to red, while &phi;=2&pi;/3 would correspond to blue, and &phi;=4&pi;/3 would correspond to green.

The RGB coordinates should be derivable from the &mu;, &sigma;, &phi; coordinates as follows:
 * $$ R = \mu + \sigma \sqrt{2} \cos \phi, $$
 * $$ G = \mu + \sigma \sqrt{2} \cos \left( \phi + {4 \pi \over 3} \right), $$
 * $$ B = \mu + \sigma \sqrt{2} \cos \left( \phi + {2 \pi \over 3} \right). $$

Hue is a coordinate (an angle of rotation) in HSL color space and HSV color space.

External info

 * Editing of hue in photography

See also:
 * Brightness
 * Bezold-Brücke shift