I can see the difference in color on your Model 39 & I can see why you would think that it is two colors. I think this is because of the Anodized Aluminum frame. It somehow affects the color since it is actually a different process than than the Blueing that S&W uses for the stainless slide. My lightweight Colt Commander exhibits some of these same characteristics. I could be wrong though. I would wait till a few more knowledgeable members chime in first before putting this to rest.
Navarre