Chief;
Yes, case hardenimg is almost universally done only on low carbon steels where a soft core with a hard skin is desirable. When the carbon content of a piece of steel reaches about 30 points (.3%) the part will through harden. It is normally heated & quenched which puts it in its maximum hardness condition & the re-heated to a lesser temperature to "Draw" the temper to its desired condition. Case-hardened parts on the other hand have their case left in virtual maximum hardness, with the core remaining soft as it did not harden. When the higher carbon steel is left in the max hardness condition it will then be too brittle & subject to cracking.
The coloring process as I understand it will work below the hardening temp so that an alloy part could be colored only, but also be aware this will leave the part basically in its annealed condition with most all of its hardness having been drawn out. I do not really know to what temp the makers of alloy steel frames draw their frames, but highly suspect they do impart some degree of hardness to them.
Any work of this nature should definitely be entrusted to someone who knows a lot more about it than I do.
The work on the gun in question does appear to be professional quality, so most likely was properly done.
