Color perception is a remarkable feat of neural computation. The physical world contains no color — only electromagnetic radiation of various wavelengths. Color is a construction of the visual system, arising from the interaction of wavelength information with neural processing that begins in the retina and extends through multiple cortical areas. This constructive nature makes color perception both a window into fundamental principles of sensory coding and a source of striking perceptual phenomena.
Trichromatic Theory
The Young-Helmholtz trichromatic theory, proposed in the early 19th century and confirmed by mid-20th century physiology, holds that color vision begins with three types of cone photoreceptors. S-cones are most sensitive to short wavelengths (around 420 nm, appearing blue), M-cones to medium wavelengths (around 530 nm, green), and L-cones to long wavelengths (around 560 nm, red). Any color we perceive can be described by the relative activation pattern across these three cone types.
Metamers: physically different spectra that produce identical (S, M, L) responses appear the same color.
Opponent-Process Theory
Ewald Hering observed that certain color combinations — reddish-green or yellowish-blue — seem impossible to perceive simultaneously, while others combine naturally (reddish-yellow, greenish-blue). He proposed opponent color channels: red-green, blue-yellow, and black-white. Modern neuroscience has confirmed that retinal ganglion cells and LGN neurons encode color in opponent fashion, computing differences between cone signals rather than transmitting raw cone responses.
The two theories are complementary rather than competing: trichromacy describes the first stage of color coding (cone photoreceptors), while opponent processing describes the second stage (retinal and thalamic neurons).
Color Constancy
One of the most impressive aspects of color perception is color constancy — the ability to perceive the color of an object as relatively stable despite dramatic changes in illumination. A white shirt appears white in sunlight, fluorescent light, and candlelight, even though the wavelength composition reaching the eye differs enormously across these conditions. Edwin Land's retinex theory proposed that the visual system estimates surface reflectance by comparing the light from a surface to the light from surrounding surfaces, effectively discounting the illuminant.
The viral photograph of a dress that some viewers perceived as blue-and-black and others as white-and-gold dramatically illustrated the role of illuminant assumptions in color constancy. Viewers who assumed the image was taken in shadow (bluish illumination) perceived gold-and-white; those who assumed artificial warm lighting perceived blue-and-black. The same image, two stable percepts — revealing how strongly top-down assumptions shape color experience.
Cortical Color Processing
Area V4 in the ventral visual stream plays a critical role in color perception. Patients with damage to V4 experience cerebral achromatopsia — a loss of color perception while other visual abilities remain intact, confirming that color is processed by specialized cortical machinery. Functional MRI studies show that V4 responds not just to wavelength but to perceived color, activating differently for surfaces that appear the same color under different illuminants.
Color Deficiency and Variation
Approximately 8% of males and 0.5% of females of European descent have some form of color vision deficiency, most commonly reduced sensitivity of L-cones (protanomaly) or M-cones (deuteranomaly). Complete absence of one cone type produces dichromacy. These conditions reveal the dependence of normal trichromatic color experience on the specific complement of cone photopigments.
Cross-cultural research on color naming has shown that while all languages develop basic color terms in a partially predictable sequence, linguistic categories can influence the speed and accuracy of color discrimination — a finding relevant to the broader debate about linguistic relativity.