The reason the frame is a different color is it is not the same stuff. Ruger casts their frames. Don't know if anyone else does that now, but for years and years nobody did. All frames were forged steel.
So Rugers have a cast steel frame, a forged steel barrel, a forged steel cylinder and an aluminum grip. Of course they are all different colors. Although the frame did start as blue, sometimes a Ruger frame will purple with age. This is considered desirable by Ruger collectors. All of 'em don't do it, for some reason.