True case hardening leaves the steel an overall grey color, as in the case of files. Charcoal, bone, and leather, and at one time cyanide, was used to impart color.
The process is pretty much the same today, using lower temperatures, so that no surface hardening occurs. The hammers and triggers of S&W revolvers were case hardened, as has been the hammers and triggers of some of my Rugers, to give a longer lasting engagement surface.
I have one Ruger Blackhawk with the hammer and trigger case hardened in 1989, and there is no indication of wear nor change since then.