I can not shed any light on exactly when the D&RGW switched from red to black, but I can attest that each of the several D&RGW 5000 series stock cars that I have worked on show evidence of red paint under the black. In many cases, the red is several layers thick and some of the cars still bear their original AC&F reporting mark and number stencils inside, indicating that most of the original structure survived the rebuild.
I had always thought that the black color came about at the same time as the rebuilds, but have no proof.
Jason Midyette