Sure wish Larry Neal would pop in here. He knows this info from the top of his head. He was the historian at the NCTM in Spencer.
The Souther Railway Historical Association"s "Ties" magazine covered much info in their March-April 1994, November-December 2010, and March-April 2011 issues. Will quote some of that info here.
Southern's president Fairfax Harrison returned from a visit in England excited about the green trains he saw.
"The Crescent Limited was inaugurated April 26, 1925. For the first fifteen months the Crescent Limited was pulled by black Ps4 locomotives. In july 1926 the new green Ps4's began to arrive on the property and the Crescent Limited received new locomotives on July 25, 1926. On October 21, 1929 three days before the Stock Market crash began, a brand-new Crescent Limited entered service. The exterior of the cars were painted in two shades of green etc. Ultimately succumbing to economics reasons June 30, 1934 spelled the death of the Crescent Limited." This is a synopsis from the 2010 issue.
Now at least the two tone green cars were named for Southern Gentleman, names submitted by the then Governor's of Virginia, North Carolina, South Carolina, Georgia, Alabama, Mississippi, and Louisiana. Five names each for the total of the 35-cars.
From some of the images in the 2010 issue the cars were a single color with "Crescent Limited" in small letters wit in the window band. Now the Crescent Limited did pull mail form NYC to Atlanta. I had presumed in standard Southern RPO cars. But in some of the images in this 2010 issue
it appears that even the RPO car is two tone.
Just a little of what I have gleamed form the "Ties."
Ron