When originally incorporated in 1870, it was D&RGRy.
Sometime in the 1880s it became D&RGRW.
Later (before 1900) it became D&RGRR.
There was a lot of turmoil through those early years. 1873 was the silver panic with it's associated economic depression. The battle with the AT&SF lasted through the 1870s, ending with the Treaty of Boston in, I think, 1881. Control of the railroad was pulled away from Palmer while he went on to build into Utah, spawning the RGW and it's associated litigation.
So there were several reorganizations through those years. Note that it was likely that some equipment got repainted before others, and you could probably see several lettering schemes in a given place at the same time.
Just my .02.