I can see the fascination of this problem. There are so many ways to approach it, from addressing the physics, to higher level attempts to get the right look. Searching around, it seems to have justifiably caught the imagination of many.
Here's another approach, made interesting by its use the connection machine [1], whereby an attempt is made to simulate the diffusion of pigments though paper.
[1] https://pdfs.semanticscholar.org/72ab/54ecaaf7458853bd441b94...