Colour Science & Perception — what the eye settles, and what it cannot Illustrated essays on colour: every swatch computed from a spectrum through a stated observer, the chromaticity diagram drawn honestly with its unreachable region marked, and every claim that two patches are identical made an assertion rather than a promise. https://www.colour-science-perception.com/ 2026-08-04T23:23:14.604Z Most of this diagram cannot be shown https://www.colour-science-perception.com/essays/most-of-this-diagram-cannot-be-shown/ 2026-08-04T23:23:14.604Z The chromaticity horseshoe is the canonical illustration of colour science, and nearly every printed copy is filled edge to edge with colours the page cannot produce. The honest version marks them, and the marking covers most of the picture. Three numbers https://www.colour-science-perception.com/essays/three-numbers/ 2026-08-04T23:23:14.604Z A spectrum has as many degrees of freedom as anyone cares to give it. The eye reports three. Everything colour science can do, and every way it fails, follows from that one collapse. A spectrum is not a colour https://www.colour-science-perception.com/essays/a-spectrum-is-not-a-colour/ 2026-08-04T23:23:14.604Z What arrives at the eye is a function of wavelength. What the eye reports is three numbers. Keeping the two apart is the single most useful habit in the subject, and almost every confusion in applied colour comes from letting them merge. These two patches are identical https://www.colour-science-perception.com/essays/these-two-patches-are-identical/ 2026-08-04T23:23:14.604Z It is the most repeated and least checkable sentence in visual perception, because the whole point is that it does not look true. Every instance on this site is an assertion in code that stops the build if it ever stops being the case. How far apart are two colours https://www.colour-science-perception.com/essays/how-far-apart-are-two-colours/ 2026-08-04T23:23:14.604Z ΔE is meant to be a distance with the property that the same number means the same perceived difference everywhere. Three successive formulae have tried, they disagree with each other by more than a just-noticeable difference, and the disagreement decides real matching questions. Seventeen observers in 1931 https://www.colour-science-perception.com/essays/seventeen-observers-in-1931/ 2026-08-04T23:23:14.604Z The standard observer that governs every colour specification in industrial use is an average over seventeen young British men, measured with equipment from the 1920s. It is known to be wrong in the blue, the correction has existed since 1951, and it has never been adopted. Two spectra, one colour https://www.colour-science-perception.com/essays/two-spectra-one-colour/ 2026-08-04T23:23:14.604Z Metamerism is usually described and almost never demonstrated. It does not have to be — the metameric black space is enormous, so a matching pair can be constructed to order, verified, and then made to come apart by changing the light. Blackbody and the colour of temperature https://www.colour-science-perception.com/essays/blackbody-and-the-colour-of-temperature/ 2026-08-04T23:23:14.604Z Heat something and it glows, in a colour fixed by its temperature alone. Planck's law gives the whole spectrum from one number, and the Planckian locus is the only curve in colour science derived from physics rather than from measurements of people. Brightness is inferred from edges https://www.colour-science-perception.com/essays/brightness-is-inferred-from-edges/ 2026-08-04T23:23:14.604Z The Cornsweet effect makes two identical regions look different by altering nothing except a narrow band at the boundary between them. Cover the boundary and the difference vanishes, which says the visual system is reconstructing surfaces rather than reading off intensities. MacAdam measured it https://www.colour-science-perception.com/essays/macadam-measured-it/ 2026-08-04T23:23:14.604Z In a perceptually uniform space the just-noticeable-difference contours would be circles of equal size. MacAdam's ellipses are neither, by a factor of eighty — and transforming them into each candidate space settles which spaces improved matters and by how much. Matching is not appearance https://www.colour-science-perception.com/essays/matching-is-not-appearance/ 2026-08-04T23:23:14.604Z CIE XYZ predicts when two lights will look the same under identical viewing conditions. It was never a model of how anything looks, and most of the confusion in applied colour comes from using it as one. Simulating what cannot be simulated https://www.colour-science-perception.com/essays/simulating-what-cannot-be-simulated/ 2026-08-04T23:23:14.604Z A colour-blindness simulation cannot show what anybody sees. What it can show is which discriminations survive, and that is a narrower claim, a checkable one, and the only one worth making. Why colour is exactly three-dimensional https://www.colour-science-perception.com/essays/why-colour-is-exactly-three-dimensional/ 2026-08-04T23:23:14.604Z Matching every wavelength with three primaries requires, for some wavelengths, a negative amount of one of them. That physical awkwardness is why the colour-matching functions were transformed into XYZ, and why the horseshoe is curved. The midpoint is not half https://www.colour-science-perception.com/essays/the-midpoint-is-not-half/ 2026-08-04T23:23:14.604Z Code 128 sits halfway along the sRGB scale and carries about a fifth of white's luminance. Half the luminance is code 188. Almost every gradient, blur and resize on the web gets this wrong, and the errors are visible once known. The display is an unknown https://www.colour-science-perception.com/essays/the-display-is-an-unknown/ 2026-08-04T23:23:14.604Z This site is displayed on the very apparatus it is about, and it knows almost nothing about that apparatus. Two figures here stop assuming and ask instead — a probe for the transfer function and a probe for the gamut. Constancy is the default https://www.colour-science-perception.com/essays/constancy-is-the-default/ 2026-08-04T23:23:14.604Z A sheet of paper looks white in daylight and white under a tungsten lamp, although the light reaching the eye differs enormously. The visual system is solving one equation with two unknowns, and it solves it by assumption. The illuminant is half the answer https://www.colour-science-perception.com/essays/the-illuminant-is-half-the-answer/ 2026-08-04T23:23:14.604Z An object has a reflectance, not a colour. The colour appears when a specified light falls on it, which is why two surfaces can match in a shop and clash outside, and why every serious matching standard names the light. What a gamut costs https://www.colour-science-perception.com/essays/what-a-gamut-costs/ 2026-08-04T23:23:14.604Z Three primaries reach a triangle and the visible region is not a triangle, so something has to give. Widening the primaries helps, has a price in precision and compatibility, and runs into a limit that is geometric rather than technological. A hex code is not a colour https://www.colour-science-perception.com/essays/a-hex-code-is-not-a-colour/ 2026-08-04T23:23:14.604Z Six hexadecimal digits identify three numbers. Turning three numbers into a colour needs a colour space, a transfer function, a white point and a display, and leaving any of them unstated is the everyday version of every confusion on this site.