This is a great paper, will likely move the state of the art forward. I've been aware of Kubelka-Munk representations of painting surfaces since [1], which used a six-dimensional space (scattering and absorption for 3 wavelength bands, not really spectral). Doing it all in RGB makes it way more practical.
I'm fiddling with an alternative approach based on IPT-like color spaces, which won't need the 10MB lookup table, and will have a tunable parameter for paint opacity (between watercolors and pastels). So far I have promising but not compelling results. If people are working on, say, a free software painting application, get in touch.