That is my question, MS. In terms of effect on the steel, the colors are mostly a by-product. You can color the steel without case hardening. Equally, you can case harden without getting colors. Are you saying that the desire for case colors is leading people to case harden parts that are of unsuitable steels just for the colors?
As far as I know, all steels can have a case of higher carbon alloy applied via the case hardening method. The more complex and higher carbon the base alloy, the more likely the extra carbon of the case layer will be of no benefit and the more likely it will lead to a crack. However, I don't see where modern medium carbon steels like 4140 would have a problem. Which "tool" steels are you refering to as being used for gun parts?
I agree that normal Rockwell testing would not give useful results on normal case hardening.
