AR receivers are investment cast aluminum with finish machining, followed by anodizing. The anodizing process grows a microscopic layer of weird crystalline oxide tubes that readily accept dye, hence the variety of colors for anodized parts. This is the way they were made until all the coating systems came to market. Mismatched upper and lower colors is a pathetic manufacturing flaw, probably harmless functionally, but poor quality control. Can you imagine a fine S&W revolver with a deep blue receiver and matte finished barrel? Same thing. All the AR's I have built have suffered this fate to some degree as the lowers, being firearms, are sourced from separate vendors from the uppers.
actually high end AR receivers are either forged or billet and finish machined...
I'm rather certain that Rugers and some others are investment cast, but I'd never knowingly purchase an investment casting, when I could have had a forging, my Rock River, and to the OP, your Smith and Wesson are forgings!