Here are two that I've had done in the past 2-4 years. I don't send them to Turnbull directly, but to Matthew Begley at Black Diamond Gun Works in Ohio. He's a terrific young double gun specialist that I highly recommend. Matthew will anneal the parts to be colored, polish to original spec and then send to Turnbull to just do the coloring. He'll then reassemble and return to you. He's far faster and, probably, overall less expensive, than having Turnbull do everything. Price for just the coloring and prep is a little tough for me to carve out because I had other work done on each of these, as well, but for just coloring I think I paid $800-$900 for what I described above.
BTW - I have no affiliation with either Turnbull or Black Diamond - just relaying my experiences. Any restoration work is not going to be a financially sound choice for a lower grade, common gun, but, in my case, they were either a family gun that I wanted to restore or one I had very little invested in that made the additional cost more tolerable. Either way, the pleasure I get from saving one of these guns that would otherwise be scrapped for parts is worth it to me! First gun is a 1930 12ga Field Ejector, second is a 1924 12ga Ideal Ejector factory 3" gun.