I have not been pulling my share here this winter, so I will put something up.
No, not that kind of coloring with crayons...
Much of this is lost to history, partially because gun companies held it "proprietary" and secret and much died with the guys doing it or records just were not saved. But here is a piece:
Trivia. I came across this on the process used by Springfield Armory for blueing (aka blackening) and "color case hardening." (Both processes were "proprietary" aka secret at say Colt and Remington and their finishes were unique to them. It is believed many of the "recipes" went to the grave with the workers.) Anyways, here is one. Parts to be blued were given a final quench in sperm whale oil. Those to be colored (CCH) were quenched in sperm whale oil but after 1878 in water. A faster system came to be used for blueing. An open cast iron vessel was built into a furnace and heated from below. A mixture of melted potassium nitrate and manganese peroxide was heated in the vessel until a pinch of sawdust dropped on the surface would burst into flame. The part to be blued was then immersed for approximately five minutes. Larger parts like barrels took a bit more. When determined to be done, the part was quenched in hot water and then dipped in an oil bath to complete the blueing. Last, it was dipped in hot water to remove the quenching oil and wiped fully dry.