How expensive is it to re harden a mauser action, worring about some of the pitting on it? Would case coloring achieve the same thing?

Pacific metallurgical is right around $150 base price. meaning send in a few actions and the cost is the same. It's a by the pound deal with a minimum price.

Color case is a bit more pricey and a wee bit thinner in case. but essentially the same thing.

Both will carberize the action then harden. and it's how the action is hardened that produces the colors.