Anyone a member of the Rio Grande Technical & Historical Society ?
Piggybacking on KD's observations, in "D&RGW Color Pictorial" by Four Ways West Publ, a number of locomotives have driver and engine truck hubs painted white or aluminum. The color appears to be the same as was used to edge the running boards.
Speculation: The Grande had two major shops, Burnham/Denver and Salt Lake City. Maybe red hubs was a practice of one of the two shops.