Though I've never mixed a "bone" color, using Createx paints, I would probably start with these:
Opaque Yellow + Opaque White or Tinting White + Light Brown
I would stay away from Black. Black is an overly strong color. Too much, or even just 1 drop, may cause your color to turn the whole batch into a dark desert tan. So not the bone color you're trying to achieve. If you do try the Black, use a swizzle stick that's an 1/8" thick, and just dip the end into the Black about a 1/8" - 1/4" and use that to mix it in. Mix it really good.
- Jeff