You can Parkerize successfully in cookware for pistol size parts. A stainless steel double-boiler works well, as does Pyrex cookware. A double boiler generally lands the solution in about the right temperature range without real effort. I had a stainless tank welded up to handle Garand barrels and op-rods with only a gallon of solution. With a 1/4 inch aluminum spreader plate on the bottom it heats very evenly. That's another consideration.
Different colors usually mean different solution base chemistry. I suppose zinc phosphate parkerizing, being very porous, might be amenable to some dies. It is often used as a paint base (take a look at the multi-color TR Gun Kote finishes (
http://tenring.com/trcoating.html)). The black pre-dips cause its crystal structure to change some, so try this kind of stuff, or even diffent brands of solution on test metal first. Also, note that color changes with the hardness of the metal. Manganese phosphate and solutions containing nickel compounds are also dark to black. The manganese is very tough and more rugged than zinc. I've always used the Brownells solutions, but have heard people are very happy with the more expensive, but easier to use Shooter's Solutions parkerizing chemistry. I bought a sample kit from Shooter's solutions of both their chemistries, but haven't had an opportunity to try them yet.
I acquired a small rotocast plastic blast cabinet for about $125. I already had a good size compressor; you need 10-15 SCFM at 90 PSI to keep up with even a small blaster if you don't want to stop and rest a lot. My only complaint about the little cabinet is its gloves come loose easily. I believe Enco carried these at one point.
Nick
Hence me asking about alternative methods.