Why does mixing all the paint colors make brown, but all the lights make white?
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Why does mixing all the paint colors make brown, but all the lights make white?
The short answer
Mixing paints makes brown because paints take colors away, while mixing colored lights makes white because lights add colors together. Paints are filters that each swallow part of the light, so stacking them leaves almost nothing but a dark muddy brown; colored lights each give off glow, so red, green and blue together fill in every color and add up to white.
How it works
There are two opposite ways to make a color. A colored light gives off (adds) its color, so it starts from darkness and gets brighter as you pile lights on. A blob of paint is a filter that swallows (absorbs) some colors and reflects back only the leftover, so it starts from white paper and gets darker as you add more paint. When you combine red, green and blue lights, their glows add up and stimulate all the color receptors in your eye, landing on white. When you combine red, green and blue paints, each one absorbs more of the light, so together they reflect very little and you see a dark brown.
What people get wrong
People often think mixing colors works the same way no matter whether it is paint or light, so all the colors together should always give the same result. In fact they are opposite games: light mixing is additive (colors add up toward white) and paint mixing is subtractive (colors take away toward dark brown). That is why the same red, green and blue can end up white in one case and brown in the other.
The catch
Lights can reach a pure bright white and any glowing color you want, but they need a dark setting and a power source, and in bright daylight the extra glow washes out. Paints work anywhere with no power, even in plain daylight on paper or a wall, but because every layer can only remove colors you can never mix your way up to white; you always drift toward dark brown.
Questions kids ask
Why do colored lights add up to white but paints add up to brown?
Lights give off color, so when red, green and blue glow on the same spot their light adds together and fills in every color, which your eye sees as white. Paints take color away by absorbing part of the light, so each paint you add removes more, and three together reflect almost nothing but a dark muddy brown.
If you keep mixing more paints, can you ever get white?
No. Each paint can only absorb more light, never add it back, so every layer makes the mix darker. You can mix your way down toward brown and near-black, but you can never mix paints up to white.
Why does a printer use cyan, magenta and yellow instead of red, green and blue?
Printer inks work by subtracting light, like paint. Cyan, magenta and yellow are the most efficient colors for subtracting, because each one removes just one of the red, green or blue parts of white light. Combining them can make a wide range of colors, and printers add a separate black ink because the inks alone make a muddy dark brown rather than a true black.
For grown-ups
This is additive (RGB) versus subtractive (pigment) color mixing. Light sources combine additively: overlapping red, green and blue primaries sum to white because all three cone types in the eye are stimulated at once. Pigments mix subtractively, each absorbing part of the spectrum and reflecting the rest, so combining many pigments leaves little reflected light and gives a dark, desaturated brown. True black is hard to reach because real pigments are not perfect absorbers, which is why printing uses cyan, magenta and yellow (the efficient subtractive primaries) plus a separate black ink.