Apophysis/Photoshop. Approx. 10 or more renders plus heavy manipulating & chaotic experimentation on how different layermodes effect on others, done mostly as a coloring-exercise. Haven´t learned as much as I did from this in a long while, so time well spent I guess. Thanks for checking out.
Have you ever been called upon to render such a thing onto a wall in large size (say, 6x8 feet), and if so, how would you attempt to perform such a replication using paint? There must be some way to do it that doesn't take weeks by hand ... like establishing the layers you spoke of, then painting them in precise blotches (outlined by hand, admittedly, from perhaps a projection) using paint that allows bleed.