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!)