Thanks Neal and Marie. I should've added - I feel like I've tried everything: rubbing alcohol, nail polish remover, oxyclean, a super-specialized dye stain remover, hair spray (it works on ink!), Mr.Clean magic eraser, bleach... I think the answer is a new skin, I was just wishful thinking.
I'm wondering if anyone has any tips on removing stains from polyurethane skins? It turns out that a pair of black shoes I have leach dye into the skin of my prosthetic. This results in my foot looking perpetually grubby. Besides replacing the shoes (which I have), any tips for fixing the dye stain that's already there?