I have one with the purple cylinder latch too. This is talked about every so often around here. If I remember right, many of the 70's / 80's Pythons latches end up turning this purple hue. Potential reasons have been talked about. Everything from the temperature of the bluing bath being a little off to maybe even that possibly the latches were made by an outside vendor for Colt.
I see them fairly regularly at gun shows, there are a lot out there that way. It does not seem to bother folks very much. Just part of the, "Python Legend".