That is beautifully colour case hardened - but - does it not effect the hardness of the action and bolt when colour cased?

The only colour cased hardening I've observed, was with the parts (in a sealed vessel with bone & other carbon items, etc, heated to brilliant red heat then dunked into a bucket of cold water. Case hardening in itself is the addition of carbon into the metal's surface through soaking in immense heat, isn't it?

I assumed the above was the "old way". How could that be safely done to an action that is quite particular to it's hardness, if done the "old way"?. Would tempering after case hardening not injur the colours?