Herb,
While I haven't seen any of these on the railroad in Salt Lake for a long time, I don't recall every seeing them painted, either here, or in Durango. I picked up a set that sounds like they belong to the same "family" of cans as yours. Exactly the same description, only two smaller sizes. Mine are not painted...but the metal is a dark grey in color.
The color coding of contents sounds like the best way to do it, but, my experience with the Rio Grande was that they were pretty consistant about using each style of can for the same contents all the time. We tended to look for the "right" can by size and shape. This of course may have varied from location to location. I'm guessing the cans that rode on the backhead were probably pretty consistant tho.
Just my two cents worth..
Doug