Not too sure about the "expert" appellation, but modern steels are through hardened, not requiring "case hardening" which is a surface hardening process. So, the case hardened colors or really decorative only.
But there is a difference between Ruger's process and Colt's, and the process Doug Turnbull uses on his restorations.
The case process used by the gunmakers, and Turnbull, is a heat applied finish using various carbon-producing material introduced during the heating process. The Ruger system is more of a dye process that does not involve use of a furnace.
Some older guns, and gun parts, are actually surface hardened, as on recent S & W action parts, which were surface hardened to reduce wear on friction surfaces.
Here is a few Doug Turnbull has done for me:
Fits my taste fine.