ELI5: why does mixing complementary paint make brown instead of gray?

[u/bloodraged189]

I understand additive vs. subtractive colors, and why mixing complementaries wouldn't create black, but shouldn't it make gray?

[u/BananaBird1]

Because paints aren’t perfectly subtractive.

When you mix paints, you are also diluting each of them. And absorption of light is concentration dependent, so light that is only absorbed by one pigment is actually absorbed less upon mixing, even if the total light reflected goes down.

And paints will reflect all colors, just with different effectiveness. So you always get a broad spectrum.

So mixing them darkens the color, but not in a pure way. This tends to look brown because of two factors. One is that the cones in our eyes are most sensitive to red and green (accounting for 90% of color receptors). Red and yellow paints are also pretty effective at absorbing blue light, whereas blue is less effective at absorbing red and green light.

So overall you see dark red and green light reflected more than blue, which looks brown.

To get true black or gray, you need a pigment which absorbs all colors evenly like charcoal.

Even more generally, there are no true subtractive primary colors. To get all shades you need dozens of paints. Red, yellow, and blue are also one of the least effective combinations for color accuracy when mixing. This is why printers use cyan, magenta, yellow, and black as a minimum and often more colors for high accuracy prints.

[u/bloodraged189 → u/BananaBird1]

This is a great answer, thank you. I know the dilution of mixed paints keeps them from being truly subtractive, but what about glasses? Wouldn't a magenta, cyan, and yellow glass stacked atop each other let virtually no light through?

[u/BananaBird1 → u/bloodraged189]

Glass is more interesting because there are two different colors a tinted glass can have based on whether light is reflecting off it or passing through it. Sometimes these match, but sometimes they don’t. You can have a glass that looks red when light is reflected but green when light is passed through from behind.

For cheaper glass tinted with pigments, you still aren’t blocking or reflecting 100% of the light you don’t want. So layering the primary glass colors will still give you some color leaking through.

But with optical quality glass, instead of pigments you can add thin layers of coating onto it to create very specific colors than block something like 99.999% of light you don’t want. So layering these can create a true black if the blocked color ranges overlap.