As I noted Ed has edited his post I assume he started it as Case Hardening, when I looked at it, it said Case Coloring. As stated the use of an acetylene torch to produce colors does not harden, but "Destroys" existing colors.
To date I have never had a frame re-hardened. The only ones I have ever re-colored were a few "Cheapies" on which I used Oxpho blue dapped & streaked on at a temp of about 200°F which does not affect original treatment. First attempt at this was on a Stavens 94 single barrel & it looked about as well as an original which would have been cyanide hardened.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra