Unique hue is a term used in perceptual psychology of color vision and generally applied to the purest hues of blue, green, yellow and red. The proponents of the opponent process theory believe that these hues cannot be described as a mixture of other hues, and are therefore pure, whereas all other hues are composite.[1] The neural correlate of the unique hues are approximated by the extremes of the opponent channels in opponent process theory.[2] In this context, unique hues are sometimes described as "psychological primaries" as they can be considered analogous to the primary colors of trichromatic color theory.[3][4]
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was invoked but never defined (see the help page).