The way I usually do this: Bestial Brown, Snakebite leather, Bubonic Brown, Bleached Bone (, Skull White according to gusto).
Sometimes I use Bronzed Flesh instead of the Bubonic Brown. Gives a less \"yellow\" appearance to the bone (see Bone Giant in my gallery).
Otherwise, like funnymouth said, look around. There are really many many ways of painting skulls... blue, grey, almost black, up to almost pure white and everything in betwwen... experiment!