It depends on the feeling you want to give the painting.
(All Paints GW)
Darker, more brooding and muted: Graveyard Earth blended up into bleached bone. Then highlight with 50/50 BB and Skull White Mix.
Brighter, cartoonish and vibrant: Snakebite Leather to BB. 50/50 mix of BB and SW highlight.
You can fiddle around with other borns but genreally the rule of thumb is brown blended up to BB, then a 50/50 BB, SW highlight in my books.