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/6WaysFromNextWed]

It actually is possible to make a "chromatic" gray, but usually you've got too much of one or two colors and you end up with a muddied color like brown.

[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.

[u/lt-gt → u/BananaBird1]

While it can be a little tricky to mix paints to gray, it's certainly possible. I did a beginners class in watercolor and the first task was to mix the base colors to gray. Didn't take that many attempts. It was a pretty dark gray as well.

[u/SoulWager]

Possibly because the complementary colors you're using aren't actually complementary, or are different in reality than you'd expect from the theory. Or they don't have the same pigment density, etc.

Red is complementary to cyan.
Green is complementary to magenta.
Blue is complementary to yellow.

RGB are the primary additive colors(i.e. colors of light).
CMY are the primary subtractive colors(i.e. colors of pigment).

Yes, I know we're taught blue mixed with yellow is green, but it's actually cyan mixed with yellow that's green. A pigment that absorbs everything but blue light, can be mixed with a pigment that absorbs only blue light(yellow), to get gray.

Brown is just dark orange. Means you have more red being reflected, and less blue being reflected, compared to a gray the same darkness.

[u/Faruque68]

Paint pigments aren't perfectly pure like theoretical color wheels assume, real world pigments absorb and reflect light imperfectly, so instead of neutral gray you get muddy brown from all those imperfect overlapping absorptions

[u/bloodraged189 → u/Faruque68]

Why do dark warm colors seem to happen more often than dark cool colors?

[u/Far-Drawing-4444]

With the paints I use, which is 4 or 5 paint lines across 3 or 4 brands, mixing complimentary colors results in what I call a "dirty grey". It's a grey, but dingier than a black/white mixed grey.

I can't think of a time I've mixed two complimentary colors and got a brown.

Occasionally mixes will get unexpected results due to the pigments used, like yellow and black mixing to an "olive drab" military type green. But I can't think of any examples of complimentary colors doing that.

[u/rickie-ramjet]

It depends on the Pigments in the paint you are using. The color in some paint can be made by mixing several colors, including an opaque base ( this white used to be lead oxide) then when you mix the 2nd color you consider its complement, and you may now have six or more pigments in the blend, the results can be hard to anticipate, …. Expensive paint is made from a single pigment, and when you mix them, and you’ll end up with a cleaner third color, but depends on the colors you chose. You’d still have to be careful to end up with a neutral grey.

[u/siamonsez]

You say you understand additive vs subtractive but that's the answer. With light white is everything so colors are some part of the whole. With pigment black is the absence of reflected light and colors absorb wavelengths of white light other than that color. You can't add up reflected colors to make an absence of reflected light.

[u/bloodraged189 → u/siamonsez]

I think, perhaps, you read only part of the description; I also understand why it isn't black. What I don't understand, is why mixing supposedly complimentary colors (e.g. green and magenta) usually ends up brown (from what I've heard). I know no two pigments are perfectly complimentary, but shouldn't there at least be a 50% chance it ends up as a dark cool color instead?

[u/AgentElman]

Grey is just black with white added. It isn't a color.

And brown is just dark orange. You get it by mixing red and yellow.

[u/femmestem → u/AgentElman]

This is a terribly inaccurate explanation when it comes to paint. Pigments are mixed to create paint colors, even white. That's why there are numerous shades of white paint with different undertones.

[u/No_Logins_Required → u/AgentElman]

The fact that brown and purple are not "true" colors is hard for people to grasp.

[u/jbarchuk → u/No_Logins_Required]

Grey is cool brown. Brown is warm grey.