All color case hardening is done using bone and charcoal. Results vary from metal to metal, but I can usually get a really beatiful and unique color and pattern by careful polishing. Cost for a receiver is $180-225 depending on the size and how much work I have to do to get it ready. Triggers and hammers run $50 each, or the two together $90.
Here's a Savage 24 .22-.410 combination gun receiver. Cost for this piece was $225
Here's a Fox Sterlingworth that got the full treatment. Color case hardened receiver and forend iron, slow rust blue barrels and stock refinished. Nice gun, I hated to send it back!
The hard part is preparing the metal without disturbing the engraving. Hand sanding is the right way.