Sorry I'm late to the party. RGS "ballast" such as it was, was usually whatever dirt the workers had at hand when the tracks were laid. (After that, there were gravel operations along the Dolores, but it's not like they spent a lot of money on trackwork.) As such, you'd have seen wide variety of colors of "ballast", dominated by the color of the local dirt: slate gray in the Mancos shale areas, which extends into Durango, red loess soils in the Mule Shoe curve area until you drop down to Hesperus, Darker organic soils up in the mountains, alternating with the kind of tannish stuff you've got here. The local gravels, when used, tend to be light gray in overall effect.

Walking tracks around Butterfly and Trout Lake in the '60s as a kid, and in the '70s - '90s over a good portion of the roadbed, there was never much evidence of real ballast except in a few select places, like Rico or Telluride yards. Here's a pic of Burns while the wreckers were ripping out track. Weed Ballast.

http://swcenter.fortlewis.edu/SWimages/P026187.jpg