Editing Opponent process (section)

===Unique hues===
[[File:Opponent colors.svg|right|thumb|240px|Opponent color pairs based on the [[Natural Color System|NCS]] experiment, including black, white and the four [[unique hues]]]]
{{main|Unique hues}}
The colors that define the extremes for each opponent channel are called [[unique hues]], as opposed to composite (mixed) hues. [[Ewald Hering]] first defined the unique hues as red, green, blue, and yellow, and based them on the concept that these colors could not be simultaneously perceived. For example, a color cannot appear both red and green.<ref name=hering1964/> These definitions have been experimentally refined and are represented today by average [[hue]] angles of 353° (carmine red), 128° (cobalt green), 228° (cobalt blue), 58° (yellow).<ref name=miyahara/>

Unique hues can differ between individuals and are often used in psychophysical research to measure variations in color perception due to color-vision deficiencies or color adaptation.<ref>{{cite journal |last1=Tregillus |first1=Katherine |title=Long-term adaptation to color. |journal=Current Opinion in Behavioral Sciences |date=2019 |volume=30 |pages=116–121 |doi=10.1016/j.cobeha.2019.07.005 |s2cid=201042565 |doi-access=free }}</ref> While there is considerable inter-subject variability when defining unique hues experimentally,<ref name=miyahara>{{cite journal| pmc=1404500 | pmid=15002843 | doi=10.2466/pms.2003.97.3f.1038 | volume=97 | title=Focal colors and unique hues | journal=Perceptual and Motor Skills | pages=1038–1042 | last1 = Miyahara | first1 = E. | year=2003 | issue=3_suppl }}</ref> an individual's unique hues are very consistent, to within a few nanometers.<ref>{{cite journal |last1=Mollon |first1=J. D. |title=On the nature of unique hues |journal=John Dalton's Colour Vision Legacy |date=1997 |pages=381–392 |url=https://www.researchgate.net/publication/265079435}}</ref>