I use London Grey and Space Wolves Grey 50/50 (although I have been experimenting with different greys, such as shadow grey). Then I add either white or bleached bone to this for several layers (up to white). Then, when dry, I give the areas several very thin washes with bestial brown and dark flesh, and sometimes vermin brown. Sometimes I sprinkle salt on the wash, let it dry, then brush it off (trick I learned in White Dwarf).