There is no colour in the world. This is one of the stranger truths that physics offers. The objects around you do not have the colours you see. They have surfaces that absorb some wavelengths of light and reflect others, and those reflected wavelengths are detected by the cone cells in your retina and translated into electrical signals that your brain interprets as colour. The redness of red is not in the apple. It is in you.
That is simultaneously deflating and extraordinary. Deflating, because it suggests the vivid particular colour world we inhabit is in some sense a construction, a film the brain projects onto a reality that is, at the level of physics, colourless. Extraordinary, because it means colour is fundamentally a relationship: between the surface properties of an object, the quality of the light illuminating it, and the specific biological apparatus of the observer.
Colour is not a thing. It is an encounter.
The physics
Visible light occupies a narrow band of the electromagnetic spectrum, wavelengths from roughly 380 nanometres at the violet end to 700 at the red. Beyond those limits in both directions lies light we cannot see: ultraviolet and infrared, X-rays and radio waves, all propagating at the same speed, all governed by the same physics, all invisible to us.
Within the visible range, different wavelengths stimulate different combinations of the three types of cone cell in the human retina, tuned to peak sensitivity in the short, medium and long wavelength ranges. The brain interprets these differential responses as colour.
This is why colour is inherently three-dimensional. Any colour can be described by its position in a three-dimensional colour space, and any two stimuli that produce the same ratios of cone response will appear identical even if their spectral compositions are entirely different.
That phenomenon is called metamerism, and it has a practical consequence that anyone who has matched paint knows: two samples that match perfectly under one light source can diverge visibly under another. Strictly, that divergence is illuminant metameric failure, and it is the reason you take the swatch to the window.
Colour harmony
The formal study of colour relationships runs from Newton's colour wheel, through Goethe's Theory of Colours of 1810, through the systematic teaching of the Bauhaus, to the computational colour systems of contemporary design.
Complementary colours, opposite each other on the wheel, create the highest contrast when placed together, each making the other appear more vivid by simultaneous contrast. The Impressionists exploited this systematically. Monet's orange haystacks against violet shadows and Van Gogh's yellow stars against the blue-violet sky of The Starry Night are not naïve choices. They are the deliberate application of a principle.
Analogous colours, adjacent on the wheel, create harmony through similarity: a unified colour temperature producing calm and coherence.
Split-complementary, triadic and tetradic schemes each represent a different geometry of the colour space, and each produces a characteristic emotional register.
Josef Albers, in Interaction of Color in 1963, demonstrated through a series of elegant exercises that what a colour looks like is almost entirely determined by what surrounds it. The same grey appears darker against white and lighter against black. The same hue appears to shift in temperature against a cool or a warm neighbour.
Colour does not exist in isolation. It exists only in relationship, and its apparent properties are properties of that relationship rather than of the colour itself.
The psychology, carefully
Colour produces emotional and physiological responses that have been extensively studied, and the picture is considerably more complicated than the colour-emotion charts filling design textbooks suggest. The full case is set out in the companion article on colour psychology; the short version follows.
Red is associated across many cultures with urgency, danger, passion and appetite, and does produce measurable physiological arousal. But the specific associations are culturally modulated. Red is good fortune in China, mourning in parts of southern Africa, danger on a western traffic light, passion in the Western romantic tradition. The emotional meaning of colour is not universal.
Blue is consistently rated across cultures as the most liked colour and is associated with calm, trust and competence, which is why it dominates the branding of financial institutions and technology companies. It also reads as cold and distant, which is a real perceptual effect of the short wavelength end of the spectrum and its association with shadow and atmospheric distance.
You will often see the further claim that blue suppresses appetite, usually supported by the observation that few foods are naturally blue. That claim is not well supported by the evidence and rests largely on a single much-repeated anecdote. The companion article deals with the appetite literature in more detail, and the conclusion there applies here.
Green occupies a peculiar position: associated with nature, freshness and health, but also with envy, poison and the uncanny. The cultural history of green pigments is a history of toxicity. Emerald green, the vivid mid-Victorian pigment that coloured wallpaper, fabric and artificial flowers, was copper acetoarsenite, and Scheele's green before it was copper arsenite. Both are arsenical, and both have been implicated in the slow poisoning of the Victorians who lived surrounded by them.
Colour in practice
The designer's relationship to colour is both technical and intuitive.
Technical. Colour spaces and profiles. The difference between additive mixing, in light and on screens, and subtractive mixing, in ink and paint. The behaviour of colour in print against screen. And the accessibility implications of colour choices for people with colour vision deficiency, which affects roughly eight per cent of men and half a per cent of women to some degree.
Intuitive. The developed sense, refined through looking and practice, of when a combination is right: when the palette creates the emotional temperature the work requires, and when the relationships between colours produce interest without noise.
The most reliable way to develop the second is to look at colour with sustained attention. Study how the great colourists worked, what relationships they preferred and why, how the colour choices in a film or a painting create the emotional world of the work. Then experiment, repeatedly, until the eye begins to trust itself.
Colour is not decoration. It is structure, it is emotion, and it is the invisible architecture of how a visual work feels.
Sources
- Newton, I. (1704). Opticks.
- Goethe, J. W. von (1810). Zur Farbenlehre / Theory of Colours. Available via Internet Archive.
- Albers, J. (1963). Interaction of Color. Yale University Press. Still the single most useful book on the subject.
- Itten, J. (1961). The Art of Color. The Bauhaus systematisation.
- Fairchild, M. D. Color Appearance Models. The standard technical reference on metamerism and colour spaces.
- Colour Blind Awareness for prevalence figures for colour vision deficiency.
- Web Content Accessibility Guidelines (WCAG) for contrast requirements in practice.
- On arsenical green pigments, see Hawksley, L. (2016). Bitten by Witch Fever: Wallpaper and Arsenic in the Victorian Home. Thames & Hudson.
Related: [Colour Psychology: What Colours Do to Us]; [The Palette]; [Sacred Colour]; [How to Read an Esoteric Claim]