Painting the magma in the cracks of his skin was quite a bit of fun. I started out with a black undercoat, then put a thinned down wash of white over all of areas that I wanted to have magma in them.
After that, I did a some wet blending with the base of his fiery mohawk as the lightest point.
It really looked funny until I went back and reclaimed all of the black areas on his shoulders. A finishing touch was added with some red washes (it helped the black look a little red even).