Colour Mixing: Why Light and Paint Don't Mix the Same Way

Two completely different ways to make new colours

Mixing colours works one way with light and a genuinely different way with paint, and the reason is almost never explained clearly: light mixing is additive, paint mixing is (approximately) subtractive, and those two words describe opposite physical processes, not just different colour wheels.

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Additive: starting from darkness and adding light

A screen pixel starts completely dark (no light at all) and adds red, green and blue light on top of each other. Each colour channel adds its own wavelengths of light to what's already there, so combining all three at full strength gives you the most light possible — white. This is exactly how a phone or monitor screen works: red plus green light overlapping produces yellow, and red plus green plus blue overlapping produces white, because your eye's three types of cone cells are being stimulated in the combination that reads as those colours.

additive (light):     red + green = yellow
                       red + green + blue = white
                       no light at all = black

Subtractive: starting from white and removing light

A sheet of white paper reflects roughly all wavelengths of light back to your eye — that's why it looks white to begin with. Paint doesn't add its own light; it absorbs (subtracts) certain wavelengths from the white light hitting it and reflects the rest. Mixing two paints means combining two different sets of absorbed wavelengths, so the more colours of paint you mix, the more wavelengths get absorbed and the less light survives to reach your eye — which is why mixing many paint colours together tends to converge toward a dark, muddy brown or black rather than white.

subtractive (paint):  yellow pigment absorbs blue, reflects red+green
                       cyan pigment absorbs red, reflects green+blue
                       mix them: only green survives being reflected by both
                       mix many pigments = absorb almost everything = near-black

Why art class teaches red-yellow-blue, and why that's a simplification

Traditional art education uses RYB (red, yellow, blue) as painting's "primary colours," and it works reasonably well for intuitive hand-mixing with real pigments. But it's a practical simplification, not a fundamental physical law — the RYB model predates a rigorous understanding of pigment absorption spectra, and it doesn't actually explain or predict the exact reflected colour of an arbitrary pigment mixture the way that treating pigments' absorption spectra directly does. Modern colour-accurate printing instead uses CMY(K) — cyan, magenta and yellow, plus a separate black ink — because those three specific pigments (each absorbing almost exactly one of red, green or blue) combine far more predictably and cover a wider, more accurate range of printable colours than the traditional RYB set.

Why your eyes are the real colour-mixing engine

None of this mixing is happening "out there" in some absolute sense — it's happening in your retina. Human colour vision relies on three types of cone cells, each most sensitive to roughly red, green or blue wavelengths, and every colour you perceive is really your brain's interpretation of the specific ratio of signals those three cone types send. Additive mixing works by directly stimulating combinations of those cones with actual light; subtractive mixing works by controlling which wavelengths of ambient light are still available to stimulate those same cones after a pigment has absorbed the rest. Different physical process, same three-receptor biology underneath both.

Frequently asked questions

Why does mixing red and green light make yellow, but mixing red and green paint doesn't?

Red and green light directly stimulate your eye's cone cells in the combination your brain reads as yellow — that's additive mixing. Red and green paint instead each absorb different wavelengths and reflect what's left; mixing them typically leaves a murky brownish colour because each absorbs part of what the other would have reflected, which is subtractive mixing — a fundamentally different physical process, not just a different colour wheel.

Why do mixing all colours of light make white, but mixing all paint colours make black or brown?

Light mixing adds wavelengths on top of darkness, so combining all three primary light colours delivers the maximum possible light to your eye, which reads as white. Paint mixing removes wavelengths from white light, so combining more pigments absorbs more and more wavelengths, leaving less light to reflect — which is why mixing many paints converges toward black or muddy brown rather than white.

Is red-yellow-blue (RYB) or cyan-magenta-yellow (CMY) the "correct" set of paint primaries?

Neither is a fundamental law of physics — both are practical models. RYB is the traditional art-class simplification that works reasonably well for intuitive hand-mixing. CMY(K), used in professional printing, is more accurate because cyan, magenta and yellow pigments each absorb almost exactly one of red, green or blue light, making their combinations far more predictable across a wider colour range.

Try it live

Everything above runs in your browser — open Colour Mixing and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

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Colour Mixing: Why Screens and Paintboxes Play by Different Rules