Yes RAMS I have also noticed some plumb color. I have a DB 22 from 1979, which viewed under strong direct light (flashlight) and using a magnifying glass, exhibits the plumb coloring you describe. I should mention that this is confined to the thumb latch.
Without good (direct) lighting and a magnifying glass, it would be hard to spot. It almost reminds me of the color on the early Ruger Flat Top Blackhawk grip frame. But more subtle.
I'm not so sure that it is "turning" a plumb color but rather differences in the metal composition caused the bluing to produce a different shade.
The only Colt that I have observed the plum color on is a .32 Courier I own. It is on the alloy frame and cylinder. Always attributed this to the alloy vs. steel,but a .22 Cobra,while around 15 years or so newer,has no hint of plum. Agree about the Ruger Flattop. I had a .44 Magnum early model,while beautiful,just wouldn't shoot lead bullets. Best,or worse,plum coloring,appears on a number of Great Westerns I own. Wonder if the Dback,might not use an alloy latch?. But many say that it is the formula/age/heat of the bluing process,or in the case of the Courier,anodizing the alloy, Bud
The plum color is a result of the hot salt bluing method and if the steel is not the right alloy or is not brought to the right tempature in the bluing tanks, it will either come out plum or turn plum over time. Older Colts and S&Ws were Carbonia blued which is a type of gas flame bluing that looks more blue/gray and only gets darker as it ages. It is a bit more durable than hot or rust bluing, but it is also more costly to do a gun in Carbonia blue.