Red, yellow, and blue are the three primary colors, the painter’s building blocks. You can mix every other color out of this trio. At least, that’s what they taught me in art class: combine any two primaries, and you get a “secondary” color, which lies between the primaries on the color wheel.
Then I picked up an optics textbook, and my world was turned upside down: the true primary colors are red, green, and blue. You can mix any hue out of these three shades of light. That’s how LCD displays work: by building pixels out of tiny sources of red, green, and blue light.
Why is the painter’s color wheel different? The textbook explained that light and pigments combine in opposite ways. Superimpose a blue and red spotlight, and the resulting light will emit blue and red wavelengths. Light mixes “additively.” By contrast, mixing paints is subtractive. A patch of red pigment is red because it absorbs light of all wavelengths except the red ones. And if you combine red and blue pigment, the mixture will absorb everything that either pigment absorbs. A pigment is a trap for particular wavelengths of light, so the more pigments you put in, the more light you catch and the less can get out.
So the physicist’s color wheel differs from the painter’s because light combines additively, while pigment combines subtractively. Except… this doesn’t make any sense! Yellow and blue paints make green, right? But if paints combine subtractively, yellow and blue should actually make… black. Here’s why.
Blue pigment reflects blue light and absorbs red and green light. If you look at the physicist’s color wheel, you’ll see that red and green light combine to make yellow light:
So blue pigment reflects blue light and absorbs yellow light. Conversely, yellow pigment reflects yellow light and absorbs everything else – i.e. blue light. So blue pigment traps yellow light, yellow pigment traps blue light, and if you put the two traps together, you’ve caught all the light of the rainbow and made black!
And yet… I’ve been painting for years, and I can assure you: at least sometimes, blue and yellow do make green.
But how? Here are three reasons yellow and blue don’t always make black and sometimes make green, even though physics basically works.
1. Real-world pigments are impure.
Your typical tube of blue paint won’t perfectly absorb yellow light. Instead, it will reflect some light in the whole spectrum, and quite a lot of light in the green part of the spectrum in particular. So if you mix a pigment like this with a “cool” yellow – that is, a yellow that also reflects a fair bit in the green range of the spectrum, you’ll get a pigment that reflects quite a lot of green light. It will still be more muted than pure green, but it will be much closer to green than to black.
So your art teacher wasn’t lying to you: yellow and blue sometimes make green. Well, maybe they lied a little bit: you can’t actually make all the colors out of red, yellow, and blue. Certainly not if you only have one tube of each color. You can mix green from a cool blue and cool yellow, and orange from a warm yellow and warm red. But if you use the same tubes of yellow throughout, either your green or your orange are going to look quite a bit like brown.
2. Painters and physicists speak different dialects.
My Polish friends take offense when I call navy blue jeans “blue,” since “navy blue” is a separate Polish word. My partner keeps insisting that my blue shirt is “purple.” By what right was I assuming that my art teacher and the optics textbook meant the same things by “blue”?
In fact, as a painter, I’d call the physicist’s blue a warm blue, almost a purple. If I try mixing a paint of that precise shade of blue with a yellow, I’ll still get green, but a very impure one. So at least some of the disagreement between painters and physicists is terminological.
For the physicist, the true additive primaries are cyan, magenta, and yellow. Compare that to the most influential color wheel in art history, taken from Goethe.
If you forget about the names, Goethe’s color wheel is remarkably close to the physicist’s one. Goethe’s “red” is a cool red that is quite close to what a physicist might call “magenta;” his blue is a cool, almost turquoise cousin of “cyan.” (And his “purple” is basically the physicist’s blue.) Given the limitations of the pigments that were available in Goethe’s day, that’s really the closest you could expect him to get to the “true” subtractive primaries of yellow, magenta, and cyan.
So even if pigments combine subtractively, what the painters call “blue” and “yellow” does often combine to green. And now… time to knock down this whole house of cards. The final reason why yellow and blue don’t always make black (but do sometimes make grey):
3. Pigments don’t always combine subtractively.
What happens if you combine red and white paint? You get pink, right? But that’s not what physics tells us! White paint reflects (more or less) the full spectrum; it doesn’t trap any wavelengths. So (subtractively) mixing in white pigment shouldn’t do anything to red!
Here’s the secret: red and white make pink because most paints don’t actually combine subtractively. Instead, if you mix equal amounts of two perfectly opaque pigments, each ray of light will interact with particles from just one of the pigments. So in an opaque red/white mixture, about half of the light will interact with white pigment particles and be reflected back, and half will interact with red particles and be reflected only in the red spectrum (and absorbed in the green and blue spectrum). And that’s precisely what pink is: light with components in all wavelengths, but with more intensity in the red range.
This is so-called “additive-averaging mixing.” By contrast, if the pigment is transparent, light will pass into the paint mixture and bounce around inside it, interacting (and getting absorbed by) pigments of both colors. This gives our old friend, subtractive mixing. This is how your printer works: by combining thin, transparent layers of yellow, magenta, and cyan. It’s probably not a coincidence that Goethe’s color wheel was made with watercolor paints, which are quite transparent as well. It’s also possible to dilute oil paints and apply them in transparent glazes; if you do that, you can actually become a human printer and paint with a cyan, magenta, yellow palette!
There’s that old joke where a farmer asks a physicist for help increasing milk production. “I have the answer,” responds the physicist, “but it only works for a spherical cow in a vacuum.” Blue and yellow only make pure black when they are such spherical cows: fully transparent pigments which perfectly absorb all wavelengths except those determined by the physicist’s idiosyncratic dialect. Outside of textbook vacuums, blue and yellow make brown, grey, green, and everything in between.
If you’d like to learn how to mix colors in practice, I’m teaching a color-centered beginners’ painting course starting July 7th. You can learn more about it and sign up here. (The first session just passed, but please feel free to contact me if you’re interested; I might create a makeup session.) And if you’d like to get my essays in your inbox, you can sign up below.