[Dfariswheel]

The two major causes of plum or reddish colors on firearms are the metal itself, or an out of control hot salts bluing system.

In most cases a purple or red color is a sure symptom of a bluing system that's not under proper control.
Either the chemicals are getting old and depleted, or in most cases, the temperature or chemical mix was off.

You often see red or purple colors done by small gunsmiths who offered rebluing.
These small shops often just didn't do enough bluing to learn how to properly monitor the tanks and just didn't take the care needed to control the process.
What you often got was "off" colors of purple or mostly red colors, especially when viewed in direct sunlight.

Since all gun makers are fully aware that some steels can come out with off colors, they take extreme care not to use those steels known to give problems.

A few times this has caused problems when using a new formula steel casting, most notably with Winchester's post-1964 Model 94 rifles and some Ruger pistols.

Winchester changed much of their processes after 1964 and made a serious mistake with the receivers of the Model 94 rifles.
They chose a cast steel and failed to do adequate research.
These rifle receivers came out of the bluing operation with a horrible Red color.
The only immediate fix Winchester could do was to iron plate the receivers and then blue the iron plating.
This caused some intense stress for refinishers when the polishing operation unknowingly polished off some of the iron plating.
When they came out of the bluing tank they were a disastrous mottled Red/Blue color.

Ruger ran into the same problem with some of their cast steel receivers which came out with a purple color.
Today these Ruger's are often looked for by collectors.

Some known good steels also can get a plum or purple color.
As example you rarely see a Colt or S&W with a purple color because they use only known good steels.
When you do see a Colt of S&W with purple color this is suspected to be guns that were blued when the chemicals were about used up and weren't changed soon enough.

Another case of purple colors are the cylinder releases on some 70's and 80's Colt revolvers.
Apparently Colt had an outside manufacturer make these for them and they were made of a cast steel that often resulted in a distinct purple color only on this specific part.

So, short of a manufacturer using a new steel they haven't researched enough, the major cause of off colors is an out of control bluing operation, with temperature out of line or chemical getting used up.

Some people report a top quality gun that ages to a purple color.
This is usually thought to be a gun blued when the hot salts chemical is getting old, but not to the point where it has a definite purple color when the bluing is still "green" or new.
These pass inspection because under factory lights the color looks good.
However, as the finish ages and oxidizes it starts getting a purple color.

About the only way to get a purple gun is to deliberately run the process with the temperature too high.
The problem is, steels vary so it would be strictly a case of trial and error to get a purple color and not a ugly red color.