On ivory chips, which are engraved (scrimshawed), some collectors have used Shiva Artist's Paintstik Colors.
http://www.dickblick.com/products/shiva-artists-paintstik-colors/?ig_id=246

It would work on engraved-style clay chips, but they are not valuable enough to bother. Some would object to modifying the chips. I have used soap and other things. You rub it on all over the surface and "engravings" and then try to clean off the surface with a cloth or whatever.

Robert
(train chip is one of a kind)