Even something as simple as a proper gradient between two colors is surprisingly difficult. I think I finally cracked that one though: https://stackoverflow.com/a/49321304/5987

Color mixing is easy if you're mixing light, just add together the linear intensities. Color mixing of pigments is a different order of complexity, because you have to consider that pigments are both transmissive and reflective and there are other effects like dot gain and spectral lines to consider.