The fields
A field says what an essay is about. The order below is not alphabetical and is not a ranking: it is the order the argument builds, and each field assumes the ones above it. Light arrives, three cone types collapse it to three numbers, a measurement system is built on that collapse, and the question of whether that system is uniform is a separate one with a measured answer. Then two fields that are not downstream of the measurement at all — what the brain does to a stimulus after it has been measured, and what a scene has already done to it before it was. Last, the places the whole apparatus is known to fail.
Themes cut the other way: they say what an essay rhymes with, across fields.
What light is
A spectrum is a function, not a colour. Illuminants, reflectance, blackbody radiation computed from Planck's law, and what has already been discarded before the eye is reached.
What the eye does
Three cone types project an infinite-dimensional spectrum onto three numbers. Everything colour science can and cannot do follows from that one collapse.
Matching and measuring
CIE XYZ, chromaticity, and gamut — a system for predicting when two lights match, frequently mistaken for a system that predicts how they look.
Difference and uniformity
How far apart two colours are, whether the space you measured in was uniform, and how MacAdam's ellipses settle the question by measurement.
What the brain does
Constancy, adaptation and simultaneous contrast — the reasons a patch of known radiance still has no determined appearance.
What a scene does
A reflectance measured on a patch is a patch. Put it in a room and the light bounces, and a bounce is a multiplication — so shadows carry their own illuminant, highlights carry the lamp, and two surfaces that matched go on matching only until the second bounce.
What a camera does
A camera is three functions of wavelength collapsing a spectrum onto three numbers, which is what an eye is — and they are different three functions. Everything a photograph can and cannot establish about colour follows from that, and from the fact that silicon sees three hundred nanometres past where anybody is looking.
Where the model breaks
Seventeen observers in 1931, an error in the blue that was never fully repaired, colour vision deficiency, and the display this page is being read on.
What it takes to deliver it
A colour that stays where it was measured is nobody's problem. This is the apparatus that carries one to somebody else — ink on a sheet, a profile, a rendering intent, a proof, and a room — and every stage of it is a map that loses something, chosen deliberately.