Color vision

The ability of eyes and brains to distinguish light of different wavelengths, producing the perception of color.

Opsin: A photoreceptor protein that absorbs light and starts the visual signal; cone pigments are opsins. How each opsin's absorption spectrum is tuned decides what its cone detects.
Isaac Newton: Physicist who showed in 1666 that white light splits into a spectrum of colors with prisms. Established that colors belong to light itself, the precondition for later theories.
Young–Helmholtz theory: The proposal that the eye distinguishes colors using three receptor types. The receptor-level rival to Hering's opponent account; both proved partly right.
Color blindness: Reduced or absent ability to distinguish colors, usually from altered or missing cone pigments. Deficits reveal how many and which pigments a visual system relies on.
Color constancy: Perceiving surface colors as stable across changes in illumination. A computational job the color system performs beyond simple cone signaling.
Trichromatic theory: The theory that color vision rests on three receptor types with different spectral sensitivities. The founding account of how three cone signals span the whole color space.
Thomas Young: British polymath who proposed in 1802 that the retina contains three kinds of color receptors. Originator of trichromacy, the framework color science still uses.
Unique hues: Four colors—red, green, yellow, and blue—that appear pure and contain no other hue. Their fourness reflects the two opponent axes, not three cone types.
Red–green color blindness: The most common color vision deficiency, caused by variants of the L or M opsin genes on the X chromosome. The commonest human color vision defect, with a known genetic cause.
Retinex theory: Edwin Land's theory that the visual system computes surface color by comparing cone ratios across regions. A proposed mechanism for how the brain achieves color constancy.

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