"Plum" bluing

[btg3]

Metal oxides run the full color spectrum. Check out aluminum or tantalum oxides. The resultant color dependsom numerous factors including the base metal, the oxide thickness, ratio of oxygen to metal at the molecular level, contaminants or other material present, process used...

Gun bluing and resultant colors are similarly affected but is a subset of metal oxide processing.

 

[dagger dog]

Ruger must run batches of different parts through the bluing process and not whole guns.
I have a 2007 model Blackhawk on which the loading gate has turned plum, barrel, frame front and rear sights are still black.