colour

Colour is the aspect of objects or light that is perceived by the human eye when electromagnetic radiation in the visible spectrum interacts with the retina. In physics, light can be monochromatic or composed of many wavelengths; an object may emit, reflect, or transmit light, and the reflected spectrum determines its perceived colour under a given illumination. Hue, saturation, and brightness (or lightness) are the common attributes used to describe colour: hue distinguishes one colour family from another, saturation denotes vividness, and brightness indicates perceived intensity.

The visible spectrum runs roughly from 380 to 740 nanometres. Photoreceptor cells in the retina, called cones, come in three types sensitive to short, medium, and long wavelengths. The trichromatic theory accounts for the initial encoding of colour in the retina, while the opponent-process theory describes how colours are processed in the brain, including phenomena such as afterimages and colour constancy.

Colour representation is managed through colour spaces. Additive colour mixing uses light (RGB) as in screens; subtractive mixing uses pigments (often represented as CMY, with black as CMYK in printing). Standardized spaces such as sRGB and perceptual spaces like CIE XYZ and CIE L*a*b* enable consistent colour specification and comparison across devices.

Etymology and usage: the word colour derives from Old French couleur, from Latin color. In American English, the spelling is color. Cultural and practical contexts influence colour naming and categorization, but scientific description separates the physical properties of light, subjective perception, and representational systems.

Historically, colour studies date to ancient optics and were advanced by Newton’s experiments on light and dispersion, later refined by vision scientists seeking to model perception and colour management.