The most common reason for purple or reddish colors of bluing is the bluing process is out of control or the chemicals were getting depleted.
Out of control temps seem to be the most common reason.
Yes, heat treating and type of steels can make for odd colors, but after 150 years or so gun makers know all about that so they don't use those steels or heat treating processes on frames or major parts.
The most common place you see these colors is on re-blued guns done by a local gunsmith who doesn't do it very often.
They often allow the process to get out of control, the mix of the chemicals to change, or the chemicals to get "worn out".
Some gun makers, even very good ones sometimes let the chemicals get a little too used up before changing or may even allow the temp to get out of control slightly.
In the case of a big maker the usual odd color is a purple that may not be evident until some time has passed and the bluing has had time to oxidize.
What looks like a good blue in the factory, after a few months or years may take on a purple cast.
Often the purple or even red color isn't apparent until the gun is taken out in the sun.
Among the cases where it was the metals, were the contractor-made purple cylinder releases on some 80's blued Colt Pythons.
For a time Ruger had a problem with purple revolvers due to the new casting metals they used.
These are now collectible guns.
The post-1964 Winchester Model 1894 was a major mistake. Winchester went to cast receivers using a metal that refused to color properly and came out a weird red color.
In order to solve this until they could figure out a permanent fix, Winchester iron plated the receivers, then blued the iron plating.
The "fun" started when attempts were made by gun refinishers to re-blue them.
Not knowing about the iron plating they polished the thin plated layer off, then were shocked when the receivers came out an ugly red color.
Winchester heard about that one, which helped ruin the company.
