Greetings TinCanSue and blewbird!
I think that I can help with the upper color, but I have not run across a reference for the medium beige found below the beltline on the 1972-'73/'74 coaches. I believe that this information was originally posted to the VAC List about five or six years ago by Charlie Burke, a Parts Manager for a long-time Airstream dealership.
Upper body color: DuPont Centari #44534-A Light Beige
Lower banana skin: Lilly Coatings #7641-A Metallic Brown
(this bottom color various with different models-this
was appropriate for the 1976 Argosy, 24 ft.)
The original colors were provided by Lilly Industrial Coatings. In 1988
Airstream published a list of Dupont Centari substitute numbers. Based on
that list the Argosy colors through 1977 were:
Upper--Light Beige 44534-A
Lower--Metallic Brown 44721-AM
I don't think that Charlie Burke is an active member on this Forum, but if you happen to be a member of the VAL List I would suggest trying to post your question there as I know that he is an active participant on that list and may respond to your question with the correct lower segment color.
Good luck with your research!