That tender beading you mention is interesting, Rod. Seems that all of the RGS engines were painted that way in the 20's and 30's but it was the first thing, due to the coal dust, to turn black! Every shot I have seen of a freshly shopped engine distinctly shows the silver, or aluminum, paint on the tender beading and running board/tender floor edging. It just didn't last long with good old Hesperus soft coal!
As far as the cabooses are concerned, I think - my opinion only - that the edges only were painted aluminum and the roofs were "roofing material" in whatever color was on hand at the time. Some of these tended to be a trailer park gray or silver and others black which faded to gray, but I believe that only the edges were actually painted.