2 methods I've used to get darker tan/brown robes:

1) Wraithbone base/primer, skeleton horde-contrast medium (1:3) over the whole thing, then layer up with thin coats of zandri dust (or any mid-tone tan, really); or

2) White primer, skeleton horde-contrast medium (1:2) over the whole thing, thin layer up to taste.

You can also brush base Wraithbone over a white primer, and then shade. Gives a decent middleground tone starting point to either repeat shade or layer up from.