I suspect the difference in the colors of the butt welds along the edges of the ribands were due to how the metals were stacked in the billet. Outer edges of iron in the billets would result in a silver line, steel would cause the black....
I think a thing to consider is that the billet is generally twisted, so the steel or iron that ended up next to an eventual weld is constantly alternating. The interesting observation, is that the entire weld line not only colors uniformly, but different on different barrels.
I think Steve's right, in that the materials chosen would likely have consistently contrasting colors, but one finish may display display them a bit different than another.
The materials seem to weld very nicely, so that might imply similarity/compatibility, and there doesn't seem to be any indication of heat treatment. The color differences seem to be related to changes in the properties of the component, and maybe not the coloring of a trace element, I think as Steve noted.
