Well, I may have added rubble and carved in some cracks, but as to the painting: It was a shockingly simple technique from Dave Taylor who is wrangling all the NOCF artists.
Basecoat dark brown (GW rhinox hide)
Drybrush dark grey (VMC Dark Grey)
Drybrush medium grey (VMC London grey or GW mechanus standard grey)
Dry Brush mix of medium grey and light cream (mechanus standard grey/london grey + VMC Pale sand)
Wash thinned secret weapon Amethyst into recesses.
Ta-da!:brushwave: