encyclopedia
Colour systems — from standard to sensation
Colour as sensation and colour as number are linked by a chain of agreements, every link of which can be broken.
Editorial thesis
A colour system matters as an agreement about what counts as the same colour: without such an agreement colour can neither be transmitted nor verified.
Reader question
Why does the same file look different on two screens and in print?
Short answer
Because the numbers in a file describe a colour only together with a device profile; without one they are an instruction rather than a value.
1. Definition and boundaries
A colour system is a way of putting a set of numbers into correspondence with a colour sensation. Systems are many because the tasks differ: to measure, transmit, reproduce, or name.
2. Colour as sensation, not property
Colour does not belong to an object: it arises in the interaction of illumination, surface, and visual system. Any system is a model of that interaction rather than a description of a thing.
3. Additive and subtractive mixing
Light adds, ink subtracts. A screen emits and therefore mixes additively, print reflects and therefore mixes subtractively. Hence the mismatch between the two worlds.
4. Three channels as physiology
A three-component description of colour follows from the structure of the retina rather than from the properties of light. It is a model of the observer, fixed as an engineering agreement.
5. The standard observer
Contemporary colorimetry rests on an averaged model of human vision established by measurement. All subsequent computation depends on that assumption.
6. Device-dependent values
Numbers of the form "one hundred per cent red" describe an instruction to a device, not a colour. The same instruction on two screens gives different results.
7. The profile as translator
A profile links a device’s numbers to a device-independent space. Without a profile a file is not portable, and all contemporary colour work rests on this mechanism.
8. Gamut
Every device reproduces a limited subset of visible colours. A saturated colour available on screen is frequently unattainable in print, and the converse also holds.
9. Rendering intents
In transfer, out-of-gamut colours are brought inside: either preserving relative relationships or preserving the accuracy of individual shades. The choice is an editorial decision.
10. Metamerism
Two samples may match under one illuminant and diverge under another. It is the principal practical problem in matching materials and the reason standard viewing conditions exist.
11. Viewing conditions
Colour is assessed under specified illumination against a neutral surround. Without this, comparing samples is meaningless — the surround shifts perception more than most expect.
12. Simultaneous contrast
The same colour looks different against different grounds. This is a property of perception rather than of measurement, and it explains divergence between instrumental and visual assessment.
13. Colour naming systems
Spot-colour catalogues solve a different problem: not measuring but naming and repeating. Their value is that a number corresponds to a physical sample.
14. The swatch book as physical standard
A printed sample fades and therefore has a shelf life. A colour standard is a consumable, which is regularly forgotten.
15. Colour as brand property
Tying a specific shade to a brand requires reproducing it on every carrier: print, screen, textile, plastic. This is a problem of reconciliation rather than of choice.
16. Perceptually uniform spaces
Ordinary colour coordinates do not correspond to perceived difference. Spaces exist that are built so equal numerical distances mean equal visual differences.
17. Why this matters for interfaces
A uniform space allows scales to be built in which steps are genuinely equal. Without it a palette looks uneven although the numbers change uniformly.
18. Contrast as a measurable requirement
Accessibility requirements set a minimum luminance ratio between text and ground. It is the only part of colour work with a verifiable numerical criterion.
19. Wide gamut on screen
Contemporary displays exceed the earlier standard, and files without a stated space render oversaturated. Declaring a profile has become mandatory rather than advisable.
20. High dynamic range
An image with extended luminance range describes light rather than reflectance and therefore requires different preparation. Earlier grading habits do not work here.
21. New colour notations on the web
The current specification allows colour to be stated directly in perceptually uniform and wide-gamut spaces. It is the first substantial expansion of web colour capability in decades.
22. First turn: colour became measurable
The advent of colorimetry turned colour from a judgement of taste into a quantity that could be argued about with instruments. It is the precondition of any industrial consistency.
23. Second turn: colour management became a system
Profiles and their automatic application made portability possible without manual conversion. Complexity moved from the operator’s head into infrastructure.
24. Third turn: the screen overtook print
Displays began reproducing colours unattainable on paper. Print became the limiting link for the first time, and the logic of preparing materials changed.
25. What is usually misunderstood
The error is to think the numbers in a file specify a colour. They do so only together with a profile; a file without one has no defined colour at all.
26. Counterargument
The objection is that all this precision is excessive: a viewer does not compare an image with a standard and does not notice deviations.
27. Where the counterargument holds
It holds for a single image without comparison. Absolute accuracy is genuinely unnecessary there, and approximate agreement suffices.
28. Where it fails
It fails when colour is an identity or a specification: a brand shade, a fabric sample, a catalogue entry. There a discrepancy means a returned order.
29. Link to materials
The same colour on matt and gloss surfaces is perceived differently, because the proportion of specular reflection differs. Colour cannot be discussed apart from texture.
30. How to read the subject today
Ask not for a colour number but for the space and the viewing conditions. A number without those two pieces of information is not a claim that can be checked.