If you want to go back to the original color scheme, I found the paint codes in the Airstream on-line library. However, they only start with the '75 color schemes. That said, it is the same one that is seen on all the others. The library lists the Argosy Travel Trailer color schemes for 1975-1978 as:
Upper...Dupont Centari, 44534-A, Light Beige
Lower...Dupont Centari, 44721-AM, Metalic Brown
If you give these numbers to your supplier, they should be able to mix an accurate color for you.