I'm not sure where you live, but here on Kauai we have very red soil. I
have soaked bisque fired pieces in a bucket of very wet mud and then
lightly rinsed them. The result has a very nice "antique" feeling, like
old bone or ivory. Its perfect for small statues or primitive fetishes.
The more dirt you leave on the darker the stain. For a darker stain, you
might try soaking the pieces overnight in a bucket of very strong tea.