r/ColorTheory

https://preview.redd.it/yshjmq3d0iyg1.png?width=1080&format=png&auto=webp&s=8b5eb4007e33743ffa94b6a0b9fe641226d9656a

Every digital color tool I've used mixes colors by averaging RGB or HSL channel values. Take a blue (say, #1A3F8C) and a yellow (#D4A017), average the channels, and you get a muddy mid-tone with some green in it. Looks plausible on screen. But it has almost nothing to do with what happens when you actually mix ultramarine and cadmium yellow on a palette.

RGB treats each color as three numbers. Mixing means interpolating those numbers. But a real pigment isn't three numbers. It's a spectral reflectance curve across the visible range, roughly 380 to 730 nm. Ultramarine blue reflects strongly around 450 nm and absorbs most of the middle and long wavelengths. Cadmium yellow reflects from about 530 nm upward and absorbs the short wavelengths.

When you physically mix them, each pigment keeps absorbing. The ultramarine still eats the reds and yellows. The cadmium still eats the blues. What survives is a narrow band around 500-530 nm where neither pigment absorbs very strongly, plus a lot of overall absorption. So you get a dark, desaturated greenish color. Not bright green, not teal. Olive, closer to what you'd actually see on a mixing palette.

This is Kubelka-Munk theory in its simplest form: the two-flux model treats each pigment layer as having absorption (K) and scattering (S) coefficients at every wavelength. When you combine pigments, you're adding their K/S ratios, then converting back through the reflectance function. The math is straightforward, but the visual result is strikingly different from RGB blending, especially for saturated colors.

The reason RGB interpolation fails so badly is that it was never designed for this. sRGB encodes perceptual brightness, not physical reflectance. Averaging perceptual encodings produces a result that's "between" the two inputs in a perceptual sense, but paint mixing isn't perceptual interpolation. It's a physical process where two materials interact with light at every wavelength independently.

This difference also explains why additive and subtractive mixing produce different results even from the same two apparent colors. Mix blue and yellow light (additive) and you get something close to white, because you're *adding* spectral energy. Mix blue and yellow paint (subtractive) and you lose energy at every wavelength. Same starting colors, opposite outcomes.

I ended up building an app that does the full spectral calculation for paint mixing (Kubelka-Munk), additive light, and industrial colorant as separate modes, because they're genuinely different physical processes. It's called Chrooma Colors, on iOS if anyone wants to play with it: https://apps.apple.com/app/chrooma-colors/id6761320708

One limitation I'll flag: the two-flux K-M model assumes opaque, diffuse layers. It works well for opaque paints and most practical mixing, but starts to diverge for transparent glazes or very thin layers where light passes through multiple times. If anyone here has worked with four-flux or Monte Carlo scattering models for paint, I'd genuinely love to hear how they compare in practice.

So a book that talks about wavelengths and how they work but also the neuroscience behind how we perceive colour.

Something digestible...