I have a question; why do some of the older Ruger 77 receivers turn "plum/purple" as they age?

I have a 77R in 6.5 Rem Mag that is very "plum", while the barrel is still dark blue. I assume that they were blued at the same time and the receiver has turned color, while the barrel has remained dark blue, because of the alloy used.

I don't see any of the newer Ruger 77s turning plum color, or maybe I'm just not looking hard enough to see them.