Incolor theory,hue is one of the properties (calledcolor appearance parameters) of acolor, defined in theCIECAM02 model as "the degree to which astimulus can be described as similar to or different from stimuli that are described asred,orange,yellow,green,blue,violet,"[1] within certain theories ofcolor vision.
Hue can typically be represented quantitatively by a single number, often corresponding to an angular position around a central or neutral point or axis on acolor space coordinate diagram (such as achromaticity diagram) orcolor wheel, or by itsdominant wavelength or by that of itscomplementary color. The other color appearance parameters arecolorfulness, saturation (also known as intensity or chroma),[2]lightness, andbrightness. Usually, colors with the same hue are distinguished with adjectives referring to their lightness or colorfulness - for example: "light blue", "pastel blue", "vivid blue", and "cobalt blue". Exceptions includebrown, which is a darkorange.[3]
Inpainting, a hue is apurepigment—one withouttint or shade (added white or black pigment, respectively).[4]
Thehuman brain first processes hues in areas in the extended V4 calledglobs.[5][6]
The concept of a color system with a hue was explored as early as 1830 withPhilipp Otto Runge's color sphere. TheMunsell color system from the 1930s was a great step forward, as it was realized thatperceptual uniformity means the color space can no longer be a sphere.
As a convention, the hue forred is set to 0° for most color spaces with a hue.
Inopponent color spaces in which two of the axes are perceptually orthogonal to lightness, such as the CIE 1976 (L*,a*,b*) (CIELAB) and 1976 (L*,u*,v*) (CIELUV) color spaces, hue may be computed together with chroma by converting these coordinates fromrectangular form topolar form. Hue is the angular component of the polar representation, while chroma is the radial component.
Specifically, in CIELAB[7]
while, analogously, in CIELUV[7]
where,atan2 is a two-argument inverse tangent.
Preucil[8] describes a color hexagon, similar to a trilinear plot described by Evans, Hanson, and Brewer,[9] which may be used to compute hue fromRGB. To placered at 0°,green at 120°, andblue at 240°,
Equivalently, one may solve
Preucil used a polar plot, which he termed a color circle.[8] Using R, G, and B, one may compute hue angle using the following scheme: determine which of the six possible orderings of R, G, and B prevail, then apply the formula given in the table below.
| Ordering | Hue region | |
|---|---|---|
| Orange | ||
| Chartreuse | ||
| Spring Green | ||
| Azure | ||
| Violet | ||
| Rose |
In each case the formula contains the fraction, whereH is the highest of R, G, and B;L is the lowest, andM is the mid one between the other two. This is referred to as the "Preucil hue error" and was used in the computation of mask strength in photomechanical color reproduction.[10]
Hue angles computed for the Preucil circle agree with the hue angle computed for the Preucil hexagon at integer multiples of 30° (red, yellow, green, cyan, blue, magenta, and the colors midway between contiguous pairs) and differ by approximately 1.2° at odd integer multiples of 15° (based on the circle formula), the maximal divergence between the two.
The process of converting an RGB color into anHSL or HSV color space is usually based on a 6-piece piecewise mapping, treating the HSV cone as ahexacone, or the HSL double cone as a double hexacone.[11] The formulae used are those in the table above.
One might notice that the HSL/HSV hue "circle" does not appear to all be of the samelightness. This is a known issue of this RGB-based derivation of hue.[12]
Manufacturers of pigments use the word hue, for example, "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 forchromium,cadmium andalizarin.
Dominant wavelength (or sometimes equivalent wavelength) is a physical analog to the perceptual attribute hue. On achromaticity diagram, a line is drawn from awhite point through the coordinates of the color in question, until it intersects thespectral locus. The wavelength at which the line intersects the spectrum locus is identified as the color'sdominant wavelength if the point is on the same side of the white point as the spectral locus, and as the color'scomplementary wavelength if the point is on the opposite side.[13]
There are two main ways in which hue difference is quantified. The first is the simple difference between the two hue angles. The symbol for this expression of hue difference is in CIELAB and in CIELUV. The other is computed as the residual total color difference after Lightness and Chroma differences have been accounted for; its symbol is in CIELAB and in CIELUV.
There exists some correspondence, more or less precise, between hue values andcolor terms (names). One approach in color science is to use traditional color terms but try to give them more precise definitions. Seespectral color#Spectral color terms for names of highly saturated colors with the hue from ≈ 0° (red) up to ≈ 275° (violet), andline of purples#Table of highly-saturated purple colors for color terms of the remaining part of the color wheel.
Alternative approach is to use a systematic notation. It can be a standardangle notation for certain color model such as HSL/HSV mentioned above,CIELUV, orCIECAM02. Alphanumeric notations such as ofMunsell color system,NCS, andPantone Matching System are also used.
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