You can also airbrush them with different shades of brown and wipe them down with a rag dipped in lacquer thinner. That will leave the color in all the natural grooves and scratches copied from the original set of tusks.
I'm no expert, and don't normally post advice like this. But, I remember wanting to do the same thing and thinking how hard it would be to do. Then when I saw a set colored that way, I was shocked at how easy it was
