I would absolutely never have a gun case hardened unless it was someone like Doug Turnbull and then you will pay dearly for it. I had a nice Fox re casehardened by someone who supposedly knew what they were doing. He ruined the gun. NIckel, hard chrome or case coloring perhaps but never case hardening.
While coloring is the result of true case hardening, some guns today are only case colored, a chemical process that does not involve the traditional heating using bone and charcoal. Ruger revolvers are an example of case coloring. If its not done right, the frame can warp, become brittle and weakened and then some day when you are standing on station 8 of a skeet field and when the smoke clears from your last shot you are left with a cracked frame. This is another, don't ask how I know this situation. Stick with Turnbull and you'll be okay. Anyone else is suspect no matter how many they say they have done