Denny,
Look at any of the existing GM documentation (resto pack, factory assembly manual, master parts manual, etc.) and you will see the lists of the RPO colors. Basically, for all of the 53 and earlier Advance Design trucks, you could buy your truck with the standard color (green or black) or choose one of the factory RPO colors.
For large, commercial, fleet orders, GM would paint the trucks however the customer ordered, including two tone. I doubt that individuals could get a custom, two-tone paint job from the factory, though a dealer might agree to custom paint a truck for a customer after they received it from the factory.
