Very easy to determine the heat used on the example, as the colors are simple steel temper colors (the Tincture of Benzoin produces no colors, only acts as a protective barrier for oxidation).
The straw/tan/bronze colors fond on the receiver means that that portion of the receiver was heated to 460-500. Purples occur at temps 530-550 deg F, blues 560-630 deg F, and Reds 1000+ deg F.
There is also a corresponding change in Rockwell hardness associated with these temperatures, as great as 20 points on the Rockwell scale.
So this so called “Low heat “ process is really not all that low of a heat, and yes, it does change the hardness of the underlying steel.
The really scary part is that since its done is spots, the hardness is not uniform across the surface like you would find in true Case Hardening
