Color case hardening is about as much an arcane art as a skill.
You may get someone to do the work, but you'll often be disappointed in the quality of colors, and you may find your frame warped.
If you're willing to spend the money of a color case job, it probably won't cost that much more to get it done by a known quality shop.
Some who are well known for the quality of color case hardening are Doug Turnbull who are world famous, Ron's, and The Custom Shop.
Turnbull is world famous for restoration and is very well known for being able to do the best color case work. Turnbull has or does do color case work for Colt Firearms.
NOTE: The Custom Shop are NOT affiliated with the Colt Custom Shop and are simply listed by Colt as a "possible" source of repairs and refinishing.