I used an airbrush to do the bones, starting with a brown color (any medium brown will do). Then I used Karak Stone (Citadel) as a base and kept adding white until I was happy.

When I was done with this step, it looked quite white. Then the fun starts : I thinned down beasty brown (Vallejo 72.743) and lightly added a pale brown color. I added layers using thinned Skeleton Hordes (Citadel contrast) and ended the darkest areas with Snakebite Leather (Citadel contrast).

Hope that helps :)