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.