Draft:Psychological primary colors

Psychological primary colors or Psychological primary colours are known as the colors proposed according to the theory of Ewald Hering. This theory argues that there are 4 primary colors grouped in opposite pairs Red-Green and Blue-Yellow, sometimes another opposite pair White-Black is included, being 6 primary colors. Ewald Hering also described the colors Red, Yellow, Green and Blue as unique tones.

Opponent process theory

Main article: Opponent process

Approximations within the sRGB gamut to the "aim colors" of the Natural Color System, a model based on the opponent process theory of color vision.

The concept of certain hues as 'unique' came with the introduction of opponent process theory, which Ewald Hering introduced in 1878. Hering first proposed the idea that red, green, blue, and yellow were unique hues ("Urfarben"), based on the concept that these colors could not be simultaneously perceived. These hues represented the extremes of two perpendicular axes of color: a red-green axis and a blue-yellow axis. While this theory with 4 unique hues was initially considered contradictory to the Young-Helmholtz trichromatic theory's three primary colors, the two theories were reconciled theoretically by Erwin Schrödinger and the later discovery of color-opponent cells in the retina and lateral geniculate nucleus (LGN) related the two theories physiologically.

Unique hues

Opponent color pairs based on the NCS experiment, including black, white and the four unique hues

Main article: 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. 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).

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. While there is considerable inter-subject variability when defining unique hues experimentally, an individual's unique hues are very consistent, to within a few nanometers.

Complementary colors

Main article: Complementary colors

When staring at a bright color for a while (e.g. red), then looking away at a white field, an afterimage is perceived, such that the original color will evoke its complementary color (green, in the case of red input). When complementary colors are combined or mixed, they "cancel each other out" and become neutral (white or gray). That is, complementary colors are never perceived as a mixture; there is no "greenish red" or "yellowish blue", despite claims to the contrary. The strongest color contrast a color can have is its complementary color. Complementary colors may also be called "opposite colors" and are understandably the basis of the colors used in the opponent process theory.

Color blindness

Main article: Color blindness

Color blindness can be classified by the cone cell that is affected (protan, deutan, tritan) or by the opponent channel that is affected (red–green or blue–yellow). In either case, the channel can either be inactive (in the case of dichromacy) or have a lower dynamic range (in the case of anomalous trichromacy). For example, individuals with deuteranopia see little difference between the red and green unique hues.