Allen, As Mark states correctly the color is a result of the hardening process albiet done in such a manner to produce the wonderful colors that are expected on these old guns. In an industrial setting where case hardening is done for just that purpose colors are not a primary concern. An example of this would be case hardening cold rolled steel. The steel is brought up to the correct temp and then dipped in cyanide. The finish from my experience is the color grey.
The only two things that I can see happening to an alloy that wouldn't harden would be that it wasn't brought up to the correct temperature or the wrong steel was used and it didn't contain enough carbon.
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"Much care is bestowed to make it what the Sportsman needs-a good gun"-Charles Parker
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