Don't know about standard gauge (did that really ever catch on?) but regarding the narrow gauge I got this last night from Jerry Day:
"Prior to 1940, most of the OA-OZ series was painted box car red with white lettering. Some of the equipment was painted black with white lettering including: Jordan Spreader OU, Ditcher OW, Ditcher OX, and Derrick OZ. On December 11, 1939, D&RGW headquarters issued an order that all work equipment was to be painted gray with black underframes and trucks. As was the case with most repaintings, the order was not implemented immediately. Some in this series did not receive the new gray and black scheme until the 50's and some were never repainted before retirement (OA, OE, ON, OQ, OR, OS, OW, OX, and OZ)."
I'd imagine the December '39 order would cover both narrow and standard.
Mike