In article , Harold and Susan Vordos says...

Controlled coloration was often practiced in heat treatment. S&W, for
example, used to color the hammer of their hand guns in that fashion. You
can expect a wonderful range of colors, blues, greens, reds. It's
chemically induced. I have no information on the process, but one of Guy
Lautard's (sp?) Bedside Readers has a formula contained within.

Starrett also does so-called 'color case-hardening' of their tools.

This was "The Bullseye Mixture" story in the bedside reader.

The idea is you pack harden items, and then dump them right out
of the retort into brine with oil in it, agitated by air pumped
through bubblers in the bottom.

Jim


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