Run your VIN through compnine - it costs a few bucks - or have the dealer do it. They both pull from the same info. Older trucks aren't in the database so depending on year you may try the dealer first since that'll be free. If it pulls up you'll have all the RPO codes and the color will be in there.
You can also try checking the paintscratch website; they're generally OK about having the correct color codes available for your year/model. Of course the on-screen representation won't be accurate but you can see what was available that year and the codes for them, and usually be able to tell which one it is.
Richard