I do not have the credentials Marc does, but worked for 35 yrs as a machinist & was around a considerable amount of heat treating & took a short course on it during my apprenticeship. Case hardening is for use on low carbon steel, through hardening is used on higher carbon steel. The characteristics of the outer case can only be accheived by leaving the case in a hard condition, ie drawn at a very low temp. This low a draw on a high alloy steel is quite apt to leave it in a brittle condition. "ANY SMITH" who would case-harden a modern action with higher alloy steels I would not allow to even make me a firing pin, as they have no regard for safety. You may well be confusing artificial chemical coloring for true case-coloring in some of these instances. There are modern makers who will put a Faux-Color on a modern alloy action.
If you closely read Marc's comments I believe he was not referring to the color hardening of the vintage guns as a "Fad" but rather the current abbuses of the practise by many. It is I think wise to note that many of the vintage makers did not recommend "Re-Casing" their own guns.
