Doombeard:
heres a brief tutorial on how i painted the lava bases 
STEP 1 started with a base coat of either scab red or red gore either is good , in this case its scab red over a chaos black undercoat - the paint pot next to it shows the next colour i added to give ou an idea of contrast
STEP 2 add a little blood red to a scab red mix and water it down scab red:blood red: water 2:1:2 and paint it on in areas you want to be brighter such as rises or edges
STEP 3 water down blood red about half and half and add some more to the highlights to build them up watering down your paints is the most important bit here because it helps blend them in , dont be afraid to water it down loads and paint it over a few times until youre happy with it 
STEP 4 water down a blood red mix with a tiny dab of fiery orange to it to lighten it and apply this carefully to your highlights being careful to leave the layers beneath it still showing in places to look something like this 
STEP 5 keep adding tiny amounts of fiery orange to your mix and build up the highlights 
STEP 6 you should have something like this , its not finished but gives you an idea of how they build up looks . to finish it i would add more of these same highlights in places because theres too much dark red, and finish it off with a watered down fiery orange put very carefully in one or two places with a less watered down fiery orange as a final highlight in tiny amounts
to finish off the rocks just use a drybrush . i put codex grey on and then skull white but skull white works fine on its own depends how deep you want to get the look .