None of my smiths show a plum color, yet. However, I had a GI M1 carbine Blued (I know, they're supposed to be parkerized...mea culpa) and the lower receiver came out bright....plum. Saw another sinner with a GI UPM M1 Carbine that had also been reblued and surprise, surprise, it, too had a plum colored lower receiver! As far as we know, we have different smiths and we both asked why the plum color. Common answer: the lower receiver (trigger guard to mag well) are made of harder steel.