Well Talino, if you're talking about a Paynes gray type of color, I would be careful. I find Paynes goes too blue when mixed with only white for the highlights. I would mix a bit of raw umber with the blue-black to get a more gray-ish dark shadow tone. I would then work up the highlights using some white, but I might also work some small amounts of ochre(careful not to go too "green") or cad orange (small ammounts!) to get a more worn look from the base color. I think this would give you the base color with some slight variation in the highlights other than just straight white. I would definitely work on a scrap pallet using small amounts of paint, mixing up some experimental colors before I went to the figure with it. HTH,
Jay H.
OKC