The Ausf 'Cs' will (depending on the time frame) be either Grey or Sand.
The Ausf 'D' (the last version) will be Sand.
However, as Dave pointed ot there were an extraordinary number of two-tone & three-tone camo schemes along with whitewashed vehicles.
As we're still waiting for the first Ausf 'A' and 'B' to hit the market, what version are you building?
I would suggest a good, inexpensive guide to the 251, this is superb (and includes paint chips!)
Modelling the SdKfz 251... ...Jim