It looks like you have lava colour down, so I won't go into that, but what I did for a lava base was glue sand onto it, and while the pva glue was still wet I dragged a pin through it forming cracks all over the base. Then when it had dried, paint the cracks bright orange/yellow like lava, and the rest black. Turned out pretty well for me. It is a little bit of a pain getting in there when there is a model in the way, but its ok with patience. I look forwart to seeing the full unit!