I believe it has something to do with Rugers casting process and the sand used to do so. I'm sure someone else here has a way better understanding of it than I do and could explain it a lot better. Some turn color and some don't.
I , and many others here have purple frames , recievers , etc. and think it adds to the beauty of the piece. One of my most "purple" ones is a 77 MkII in a 7mm-08 that the reciever has turned quite bright over the years. Many of the Old Model handguns are colored as such and it is a symptom that is sought after and prized by a lot of collectors.
It is the addition of silicon (not to be confused with silicone or silica sand) in the metal mix of an investment casting. Silicon helps the metal flow into all the intricacies of mold. Too much and you get the purple color. Mixed improperly and you get purple colored lightning streaks or blotches.
It can be given a regular blue though. A friend just had completed a 2.5" 50cal wildcat on a Ruger MkII action and in bluing the barrel, the 'smith blued the action to match. Shame as it is a pretty tinge.