I just took a look at your posted photo. Yup, that's the color Ruger actions turn with a little age. It has to do with a chemical change that happens over time. It doesn't mean there is anything wrong with the metal, just a color change.

It will continue until the entire action is a uniform color and will get a bit more "Plumish" looking with time. I don't know if the color change continues or will stop at some point in time.

I've had My 458 Lott RSM for about 2 1/2 years now and it's doing the same thing. Who knows, it might even be considered a desirable trait by collectors some time in the future, naw, probably not.

Roi