Colours are perceptual categories produced when the human visual system responds to visible electromagnetic radiation and the brain interprets the resulting patterns of photoreceptor activity. In physics, visible light is electromagnetic radiation occupying approximately the wavelength range from 380 to 750 nanometres, although the exact limits vary among observers and definitions. Different wavelengths can produce different colour sensations, but colour is not simply identical to wavelength.
Spectral colours such as red, green and blue can be associated with relatively narrow wavelength ranges, whereas many perceived colours, including white, pink, brown and purple, do not correspond to a single wavelength. Colour therefore involves both the physical spectrum of light and biological perception. An object does not necessarily possess a colour independently of illumination. Its perceived colour depends on which wavelengths it reflects, transmits or emits, the illumination spectrum, the surrounding visual context and the observers visual system.
Human colour vision begins when light enters the eye and reaches the retina. Most normal daylight colour vision depends on three classes of cone photoreceptors, commonly called S, M and L cones because their peak sensitivities occur broadly toward shorter, middle and longer wavelengths. These cones do not respond exclusively to one colour. Their spectral sensitivity curves overlap substantially, meaning that a given wavelength can stimulate more than one cone type.
The brain compares the relative patterns of cone responses to infer colour. This is one reason colour cannot be reduced to a simple one wavelength equals one colour rule. Rod photoreceptors are highly sensitive to low light but provide little chromatic information under ordinary conditions, which is why colour vision becomes much weaker in dim illumination. The neural processing of colour begins in the retina and continues through visual pathways and cortical regions where information is combined with spatial and contextual signals.
A fundamental distinction in colour science is the difference between spectral colour and nonspectral colour. Spectral colours can be produced by relatively narrowband radiation at particular wavelengths within the visible spectrum. Red, orange, yellow, green, cyan, blue and violet are often used as broad names for such regions, although boundaries between colour names are not physically sharp. Nonspectral colours arise from mixtures of wavelengths and cannot be represented by one wavelength alone.
Purple and magenta are important examples because no single wavelength of visible light corresponds to them in the same way that a spectral red or green can be represented. White and grey are also mixture based perceptual categories. Brown is strongly dependent on context and luminance, illustrating that colour perception cannot be described solely through wavelength. These distinctions are essential when connecting physical spectroscopy with human colour terminology.
Colour can be produced by emission, reflection, transmission or scattering. A luminous object such as a star or LED emits electromagnetic radiation, and its spectral distribution determines the light available to an observer. An illuminated object may absorb some wavelengths and reflect others, producing a characteristic appearance under that illumination. A transparent material may transmit some wavelengths more efficiently than others, producing coloured transmitted light.
Atmospheric particles and water droplets can scatter particular wavelengths differently, generating natural colour effects such as blue skies and rainbows. Pigments and dyes produce colour primarily through selective absorption associated with molecular electronic structures. Metals can have characteristic colours because their electronic structure produces wavelength dependent reflection and absorption. Thus, the physical mechanism producing colour varies substantially between different materials and environments.
Additive and subtractive colour systems describe different ways of combining light and materials. Additive colour mixing combines emitted or transmitted light, commonly using red, green and blue primaries in electronic displays. Appropriate combinations can produce white and many other perceived colours because the combined light stimulates the visual system in different proportions. Subtractive mixing involves materials such as inks, paints and dyes that absorb portions of incident light.
Traditional printing commonly uses cyan, magenta, yellow and black inks because combinations of these absorptive materials can selectively reduce parts of the reflected spectrum. Real pigments do not behave as mathematically ideal subtractive filters, so practical colour reproduction involves complex spectral interactions. The distinction between additive and subtractive mixing is important because combining coloured lights and combining coloured pigments do not follow the same physical process.
Colour perception is strongly affected by context. The same physical stimulus can appear different when surrounded by different colours, luminances or spatial patterns. Colour constancy allows the visual system to maintain relatively stable interpretations of object colours under changing illumination, even though the actual spectrum reaching the eye may change substantially. Simultaneous contrast can cause a neutral region to appear tinted when surrounded by a strongly coloured region.
Adaptation can also alter perceived colour after prolonged exposure to a particular stimulus. These effects demonstrate that colour is not a passive readout of wavelength. The brain interprets visual signals using comparisons and contextual information. Colour science therefore distinguishes physical quantities such as spectral power distribution and radiance from perceptual quantities such as hue, saturation and lightness.
Colour can be described quantitatively using colourimetry. The CIE colour systems use standardised colour matching functions derived from human visual experiments to represent visible stimuli numerically. A spectral power distribution can be transformed into tristimulus values, which can then be represented in chromaticity diagrams or other colour spaces. Modern colour spaces such as CIELAB are designed to organise perceptual differences more usefully for particular applications, while RGB systems are commonly used for displays and image processing.
Hue broadly describes the perceived family such as red or green, saturation describes the degree to which a colour appears relatively pure or vivid, and lightness or brightness describes aspects of perceived intensity depending on the colour model. These terms are related but are not interchangeable with physical wavelength, power or photon energy.
Colour has applications throughout science, technology, art, medicine, manufacturing and communication. Astronomers use spectral colour information to infer temperatures, compositions and physical processes in stars and other astronomical objects. Remote sensing uses differences in reflected wavelengths to distinguish vegetation, water, minerals and land surfaces. Medical imaging can use colour coding to represent quantitative data, although artificial display colours must not be confused with physical tissue colours.
Digital cameras use sensor responses and computational processing to estimate colour, while printers and displays use different colour reproduction mechanisms. Colour standards are important in manufacturing because small spectral differences can affect product appearance. The study of colour therefore spans optics, electromagnetism, quantum physics, chemistry, neuroscience, psychology, computer science and design. The central principle is that colour is a perceptual interpretation of spectral information rather than a simple physical property attached permanently to every object.
