Very cool.
Colors are mixed internally in a four-component pigment space instead of RGB. To accelerate the conversion back to RGB for realtime painting, two 48 MB look-up tables are used.
The choice of these four primary pigments seems like a crucial artistic choice, because any color you choose on a traditional RGB/HSV picker is reinterpreted as a combination of these pigments. Seems like there might be an opportunity here to rethink the traditional color picker.
This algorithm is apparently implemented in a painting app called Rebelle. I haven't heard about it before, but I think I'll buy a license because it looks awesome: