I don't think it's purely coincidence that Parker, Colt and Winchester, all Connecticut companies had such beautiful colors in their case hardening process. I had an order to produce twenty four 410 Stainless steel packing boxes for Colt Mfg Co. I was asked to come over to their plant, only a few miles from my foundry, to go over their design before we made the pattern equipment. I remarked that I had several first generation single action army revolvers that still exhibited wonderful color and was told they continued to use the same packing materials in their process, developed in the 1860s, with the exception they no longer used dried horse 'road apples'. That was as far as they would go.
The attached picture is of a documented gun, issued to E. Burke, Company E, 1st Volunteer Regiment. It was returned to Colt for conversion to Artillery configuration in 1901, and never reissued. It was re-case hardened at that time.

Also, as a matter of educational information to the newcomer, on the Parker hammer guns after considerable use to the point where the color has worn off or mostly off the frame, the lock plates will usually retain significant case hardened color.
Case in point, a 1-frame C-Grade that I had several professionals work their magic on nearly every aspect of the gun, with the exception of the frame… I wanted to leave the frame untouched because of the subdued beauty of the lock plates with some color remaining from when the gun was manufactured.


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