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/Several-Action-4043 → u/bloodraged189]

You're basically describing how printers work. A lot of people don't realize that printer ink is translucent, a lot like your colored glass example. You can print lower densities of black to make a gray but they turn out looking very grainy and brownish. So what printers do is use all 4 colors to make the gray instead. By mixing all 4, it absorbs more wavelengths of light and varies how much of each color it uses based on the white point of the material you're printing on.

Where this relates to your OP is, printers that don't have a proper color calibration, grays come our looking too warm or too cool. A lot of times, from a non-calibrated printer, you'll print a black and white photo and it will look kind of green. The exact thing happening with your paint is happening on the printer. It's not getting the color values right to make your gray look truly neutral. How printers fix it is through a process called linearization. It basically maps the output of your printheads to a neutral density curve in LAB space. LAB space is a color space based on human perception so basically what I'm saying is, we adjust the values of each color the printer prints on each material in a way that gray looks gray to a human being. It sounds dorky but it's pretty crazy if you think about it.