It all depends on the steel and the temps. If someone treats 4140 to get "case colors" then you are operating at different temps than you operate at to get the best mechanical properties. The part would show colors that are the fad now, but the life and usefulness of the part would be compromised. If you treat 4140 to have the best mechanical properties, then you will not have the en vogue colors...but then part will be able to function with reduced chance of failure for along time.
The colors on the old doubles were just a by-product of the hardening process and were originally polished off. Then the colors became cool and folks started leaving them showing on the external metal part. But they were not color casing just for beauty's sake-it was a by-product of the treatment required to give a part the required mechanicla properties.
Some steels can be color case hardened and still be OK for certain applications. 8620 is incredibly tough when case hardened for maximum strength. It is still pretty tough when color case hardened and would work fine for lower pressure rounds. But the temps require for the operations are different, so color casing 8620 for use in an application that would use a "modern" high intensity cartridge is not a good idea. Say a single shot with an 8620 frame could be color cased and used in an old BPE cartridge operationg at the lower pressure The same frame could be colored for maximum toughness and stand up just fine to a 7x65R, but the required temps for this treatment will not going to give you the colors that are the fad these days.
