CM,

I believe you are right that coloring can be done at low temps. After all, dark blue is reached at about 600°F and all the others are reached at lesser temps.

How else could Turnbull and others color a modern receiver made from 4140 or similar through hardening alloys? Most of them run around RC 30 or therabouts which corresponds to a tempering temp of about 1000°F. The heat treating procedure for 4140 is completely incompatible with conventional color case hardening of low carbon steel. Since all the colors can be produced well below that tempering temp., there is no problem heating and quenching as long as one does not exceed that temp. I think this is how the pros do it on a modern steel gun.

OB