I have assisted my gunsmith with the bluing of many guns over the years. Occasionally a part comes out of the tanks purple. As we do the bluing in batches, I cannot attribute the color to anything other than the formulation of the steel. We may do multiple 1911 frames, and once in a while one comes out purple. The parts that can usually be counted on to turn are Essex frames and O3A3 receivers. My first Bullseye pistol that he built for me had a deep blue Remington slide and a purple Essex frame. They were both prepped and blued in the same batch. By the way, after the parts come out of the final fresh water rinse, we immediately hose 'em down with WD40. I have rifles we blued 25+ years ago that are the deep rich blue today that they were when they came out of the tanks.