I tried a few new color schemes here. I have no experience in painting bone or wood in oils, so I just guessed. I think it turned out pretty well.
For the shield, I used burnt umber for the shadows (underneath the skulls and between the planks), yellow ochre for the mid highlights and yellow ochre pale for the upper highlights.
The skulls (not finished yet) were painted using burnt umber as a shadow and blending upto titanium white. The tusks were painted with burn umber, 1:1 mix of titanium white and yellow ochre, and titanium white.