Can you elaborate on why your don't get grey? That was always what I expected and I was surprised that you just get a muddy brown if you mix random paints.
One practical reason you don’t get gray is that pigments have to be real chemicals and so you can’t have a green that perfectly balances a given red’s spectrum. For transparent inks on a white background, if their spectra did match perfectly, you would get a perfect gray. But those perfect dyes don’t exist (or are expensive), hence the K in the CMYK color model.
For opaque paint, it’s weirder: paint scatters and absorbs, so white light goes into the paint layer and starts bouncing around. The resulting reflectance spectrum you see is a function of the ratio of absorption and scattering across the spectrum. Mixing paint mixes the absorption and scattering spectra. One weird result is that adding white can increase a paint’s saturation. (Think of adding apparently-black blue food coloring to white frosting.) Weirder still is you can have two identical-looking paints that, mixed with a third paint make different shades depending on which you use. As a related weird example, yellow plus black can give you a blueish-gray, since the black paint may absorb yellow more than blue (so long as it doesn’t scatter much that’ll still look black) and the yellow will scatter some blue and not absorb all blue (it’s not perfect). So when you mix those you can get something that scatters blue more than it absorbs it and absorbs yellow more than it scatters it. But pick a different black and you’ll get a different result! Color theory is tricky stuff; paint mixing is perhaps the least intuitive part of it.
Thanks. This reminds me of the part of "Surely You're Joking, Mr. Feynman" where a painter says you can mix white and red to get yellow and Feynman says it's impossible and it turns out the painter would use a little bit of yellow to "sharpen it up" [1]. Any way that is possible without yellow paint?
Interesting. Without very odd paints, I can’t see how you’d get “yellow”, but if your red is desaturated (scattering and not totally absorbing some blue through yellow), and if your white is a bit gray, but your white absorbs and scatters red more than it absorbs and scatters the rest of the spectrum, the mixing path through Lab* color space would (I think) go from red toward light orange, toward warm light gray, finally coming into the light-gray color from the orange direction. I’m a little rusty, but I think that’s right.
Nitpick: what you are calling "saturation" corresponds better to the biconic color space coordinate most people call "chroma". Very dark blues may not be perceptively far from black, but they can have very lopsided spectra and be highly "saturated" in the sense of distance from the gray point in the LUV chromaticity plane.
Basically the counterintuitivity you're citing goes away when you talk about color distinct from luminance, which is how most people here understand it anyway.
The best explanation I've found is in the online book "The Dimensions of Colour" by David Briggs. In particular the section on "Colour Mixing in Paints" (http://www.huevaluechroma.com/061.php). You might have to read some of the preceding sections to get a complete understanding.
In practice it is possible to mix a grey from coloured paints, but you need disproportionately much blue. Brown is just dark orange, and blue is the complementary colour to orange, so you need to move in the blue direction if you want to get to grey. Also, if you're using red, yellow and blue, you generally need less yellow than red. Try a ratio of 1:2:4 = R:Y:B.
