Lots of information regarding case coloring with help from Mike Hunter and SDH
https://doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=572094

Decent overview of both color case and heat treating
https://uplandguncompany.com/color-case-hardening-and-heat-treating/

Yes, color case (surface) hardening is a form of heat treating, but not necessarily heat treating to increase strength of the forged action (as in rifle receivers).

I looked at several sources with descriptions of the Hunter Arms factory, all of which mentioned "large ovens for case hardening, bluing and annealing".