Should be a code in the glove box on the RPO list. Will be two numbers followed by a U, like 12U (white).
http://www.tcpglobal.com/autocolorli...truck-pg01.jpg
Their color chip book only goes back to 84 unfortunately, but you could stop in at a paint store and they'd no doubt have a chip book. If your local O'Reillys is also a paint carrying store, they'd have a color chip deck as well I bet.
Yours could be 87U, wheatland yellow.