posted this on the French forums so though I'd pop it up here
first up it's given a black undercoat and drybrushed. Starting with GW Charandon Granite and the working up through Codex Grey to Fortress Grey in increasingly light coats. Concentrate on the larger rocks for the final highlights
Now for the lava: These are the colours used (apart from white which I forgot to show)
They are GW Blood Red (BR), Red Gore (RG), Blazing Orange (BO), Fiery orange (FO) and Golden Yellow (GY) plus Black and White. Fiery orange has been discontinued and that's a shame 'cos it's a great colour >:(
For the base colour mix some RG with a dab on black and paint it on.
Then, while the first coat is still wet, start painting on some straight RG. Keep the paint fairly thin and sorta dab it on to get a nice, uneven texture happening.