A lot of the times with 'Eavy Metal paint jobs, it could be something incredibly complex. It also could have been painted with the new paints, as they've been in development for something like 3 years, and not the old ones.
Looking at the pic and the new line of paints from GW, I'd go with the Zandri Dust base color, darkened with a touch of a dark grey or black, then Baneblade Brown and Ushabti Bone from the layer paints. Being shadow, base color, and highlights in that order. But that's just a guess.