DRGW paint

Here's some info on the various yellows and oranges used over the years.  Both UtahRails and the DRGW Modeling and Historical Society are good resources for Rocky Mountain railroading.

http://utahrails.net/drgw/rg-diesel-paint-schemes.php

http://www.drgw.org/reference/paint2.htm

I've found that my best bet is color-matching to whatever decals I'm using.  In my case, I determined that I needed Aspen Gold, so I got some MicroScale decal sets and matched them by trials.  (I even found "pumpkin-colored" big 12 oz. spray cans at the local hardware store that were dead on:  much cheaper!)