Insert sole/soul pun here.

I've dabbled in some sole-painting in my time but to be honest it worked neither on an aesthetic nor function level - the rainbow fish were not as picturesque as I hoped they'd be, and the paint never dried. Nevertheless I was chuffed to find someone has found more success in the same endeavor:
Taylor Reeve hand paints these mamas herself and while her aesthetic isn't necessarily to my taste, the idea is pretty rad.

Not from Taylor but the tantalisingly named Etsy store, Hippy of Doom, comes a variation on the idea: