The one that was made from a buffalo leg bone in my post about "Gifts for friends" had some void and I mixed ground white pepper into the epoxy to fill it. I also added little cayenne to add some color. This was only for small voids so on the larger voids you may have to do multiple fills? I have also used the 2 part marine epoxy that comes in two film canisters(the old kind before digital) it is very heavy and thick so it won't sag much as it hardens. It mixes pretty dark so it would be good for prongs or gemsbok but not so good for lighter bone? I find it at the hardware store.
as far as mixing color into the epoxy I have used spices, saw dust, bone dust, etc.. with good results.
Bob Urban