Color mixing
A Socratic walk-through of color mixing — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does mixing paints darken a colour while mixing lights brightens it?
Shine a red lamp, a green lamp and a blue lamp on the same patch of wall and the patch gets brighter, until where all three overlap it goes white. Squeeze red, green and blue paint onto a palette and stir, and you get mud. Same three colours, opposite outcomes. Whatever colour turns out to be, it plainly is not something you can just add up — so what is actually being combined in each case?
Reasoning it through
REASONING #Take the lamps first, because they are the simpler half. Each lamp throws photons at the wall, and the second beam does nothing whatever to the first — light does not interfere with light in any way you would notice on a wall. So at every wavelength, the power arriving is the sum of the powers the lamps supply. More sources, more light, brighter patch. Adding is exactly what is happening.
Now the paint. Notice what is fixed here that was not fixed before: the illumination. The room light is already what it is, and the paint emits nothing. A pigment particle takes the light that lands on it, absorbs some of it, and scatters the rest back out. Yellow paint is yellow because it absorbs the short, bluish wavelengths and returns the rest; cyan paint returns the short and middle wavelengths and absorbs the long, reddish ones.
So what must happen when you stir them together? A photon now has to get past both kinds of particle. The long wavelengths meet the cyan pigment and are absorbed; the short ones meet the yellow pigment and are absorbed; only the middle band survives both, and the mixture looks green. Each pigment you add removes another slice of the spectrum, and nothing in the process can ever put a slice back. That is why mixtures darken and dull, and why the limit of enthusiastic mixing is a muddy near-black.
That answers the surface question — add power, or remove it — but it leaves the stranger thing untouched. Why does red light plus green light look yellow? Play two musical notes together and you hear two notes; you can pick them out. Why does the eye not simply report "red and green at once"?
Because the eye is not a spectrometer. There are three cone types in the retina, each responding across a broad band of wavelengths, and the entire incoming spectrum — a curve with as much detail as you like — is reduced to three numbers before anything reaches the brain. Three numbers is all the information there is.
And that has a consequence worth pausing on. If two quite different spectra happen to produce the same three cone signals, they are not merely similar colours. They are the same colour, with nothing left over to distinguish them by. A single wavelength of spectral yellow and a mixture of red and green light do exactly this; such a pair is called metameric.
Which is what makes a screen possible at all. Your display cannot reproduce the spectrum of a sunset; it has three emitters and nothing else. It does not need to. It only has to hit the same three numbers.
The analogy
THE ANALOGY #Imagine a long, detailed document that no one ever reads. Instead, every reader answers a three-question survey about it, and only the three answers are ever passed on. Two entirely different documents that happen to score the same on all three questions become, to everyone downstream, the same document — and any forger who knows the three questions can manufacture a match without reproducing a single paragraph.
The three "questions" are not clean or independent — cone sensitivities overlap heavily, so the answers are correlated — and the survey is not fixed: adaptation to the surrounding light and the colours next to a patch both shift what gets reported, which a static questionnaire has no way to represent.
Clarifying the model
THE MODEL #Three refinements keep this honest.
Paint is not really a stack of filters. Inside a paint layer, light scatters off particles, re-enters the film, and is absorbed on some later pass, so absorption and scattering are tangled together — which is why two blues of identical hue but different particle size mix quite differently with the same yellow. Pure subtraction is the clean skeleton of the story, not the whole of it.
Additive and subtractive are also not two kinds of colour. They are two ways of shaping a spectrum before it reaches the eye — one by summing sources, one by removing bands from a source already present. Everything after that point, in both cases, is the same three-signal channel.
And "the primary colours" is a design choice, not a fact about light. Cyan, magenta and yellow work well as subtractive primaries because each removes roughly one third of the spectrum; the red, yellow and blue taught in schools are a worse set for the same job, and no triple of real primaries reaches every visible colour. A metameric match is likewise a match for one observer under one illuminant: two fabrics that agree under shop lighting can disagree outdoors, and observers differ in the exact shape of their cone responses.
A picture of it
THE PICTURE #How to readStart at the rounded terminal at the top and follow the light. The diamond is the only branch that matters: meeting more light takes the left path, where powers add and the patch brightens, while meeting pigment takes the right path, where a band is absorbed and never returns. Both paths land on the same slanted box — a spectrum arriving at the eye — and so does the second slanted box on the right, which stands for some completely different spectrum. The point of the drawing is what happens next: both are squeezed through the three cone types into one circle holding three numbers, after which nothing downstream can tell them apart.
What became clearer
WHAT CLEARED #Lights brighten because beams superpose and their powers sum; paints darken because each pigment can only remove another slice of a fixed illumination. But the deeper answer is that colour is not a property of light at all. It is a three-number code the retina computes, and once two different spectra produce the same code they are the same colour, permanently and without remainder. Screens, printing and paint mixing are all just different ways of manufacturing the code you want.
Where to go next
ONWARD #- How colour constancy keeps a white shirt looking white under candlelight and daylight, despite quite different spectra.
- Why colour blindness is best described as losing a channel rather than losing particular colours.
Key terms
TERMS #| Term | What it means |
|---|---|
| Additive mixing | combining light sources, so spectral powers sum and the result is brighter. |
| Subtractive mixing | combining absorbers in a fixed illumination, so each removes a band and the result is darker. |
| Metamer | one of two different spectra that produce identical cone signals and so look identical to a given observer under a given light. |
| Trichromacy | the reduction of an entire spectrum to three signals by three cone types. |
Every term the collection defines is gathered in the glossary.