Hue

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This article is about the gradation of color. For the city in Vietnam, see Huế. For other uses, see Hue (disambiguation).

An image with the hue altered

The hue of this Painted Bunting is adjusted over the spectrum.

A hue refers to the gradation of color within the visible spectrum, or optical 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 —one without added white (tint) or black (shade) pigment. A hue is a color on the Color wheel.

In an RGB color space, hue can be thought of as an angle φ in standard position. To calculate φ, let R, G, B be the color coordinates in RGB space, defined on a scale from zero to one. Then, after obtaining the brightness μ and the saturation σ, the hue could be obtained from

$\phi = \arccos \left( {R - \mu \over \sigma \sqrt{2}} \right)$

(Compare with standard score). Using this formula, φ = 0° [0^c] would corresponds to red, while φ = 120° [2π/3^c] would correspond to blue, and φ = 240° [4π/3^c] would correspond to green.

The RGB coordinates should be derivable from the μ, σ, φ coordinates as follows:

$R = \mu + \sigma \sqrt{2} \cos \phi,$

$G = \mu + \sigma \sqrt{2} \cos \left( \phi + 240^\circ \; \lbrack {4 \pi \over 3} ^c \rbrack \right),$

$B = \mu + \sigma \sqrt{2} \cos \left( \phi + 120^\circ \; \lbrack {2 \pi \over 3} ^c \rbrack \right).$

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

Manufacturers of pigments use the word hue e.g. 'Cadmium Yellow (hue)' to indicate that the original pigmentation ingredient, often toxic, has been replaced by safer (or cheaper) alternatives whilst retaining the hue of the original. Replacements are often used for Chromium, Cadmium and Alizarin.