Stuart:
This question along with the tricolor heralds on ng engines has been discussed (cussed) and argued here for years. My only input comes from looking at color slides taken by Dick Kindig going back to 1937. Dick photographed every D&RGW ng engine in both color and B&W. He had a camera setup with a Leica 35mm camera mounted on top of a medium format camera loaded with B&W film. There was not one ng engine with green paint until some were given green by the Salida shops for Rocky Mountain RR club excursions. As some have said, never say NEVER, but in looking at hundreds of color slides taken from 1937 on, you would think there would be at least one photo if any of them were in green and there isn't even one.