I have never seen a Winchester model 12 nickel steel gun that Simmons blued turn purple over time. There is a technique of salts/temperatures that results in a black that lasts. Not too many people know how to do that anymore. Different alloys used in different gun parts need different times and temperatures to come out black and stay black. A lot of guns and parts present bluing problems.
And as stated, a rust blue will not turn colors, at least as far as I have ever seen.

Dennis Potter