This link explains the origin of the green paint. It lasted on most engines until they were retired 1948-53. In the 1980s I rode and chased Southern engine 4501 on fantrips and I heard the green paint called Virginia green several times by car host and railroad club members. From the Southern Railway PS-4 page that I can't link to for some reason:
The queens of steam locomotives[edit]
In 1925, Southern Railway president Fairfax Harrison traveled to the United Kingdom, where he admired the country's London and North Eastern Railway's use of green-painted steam locomotives.[3] In 1926, Harrison's trip had inspired the appearance of the second order of Ps-4s, which was being built by ALCO's Richmond Works at the time.[3] These Ps-4s were painted in Sylvan green with gold leaf trimming and lettering.[3]
This order consisted of eleven locomotives for the Southern, numbered 1393-1404; seven for the Cincinnati, New Orleans and Texas Pacific, numbered 6476-6482; and four for the Alabama Great Southern, numbered 6688-6691.[6] Aside from the paint scheme, which would soon be applied to all of Southern's passenger locomotives, the second order had other notable differences.[6] They featured an Elesco feedwater heater rather than the Worthington heaters of the previous order, with the former placed on top of the smokebox between the stack and bell instead of under the running boards as the latter were placed.[6] The second order also had larger tenders better suited for long-distance passenger runs, with three-axle bogies and 14,000-gallon water capacity, versus the two-axle bogie, 10,000-gallon standard USRA tender design of the first order.[3]
The final Ps-4s were built in 1928 by the Baldwin Locomotive Works, consisting of only five engines for the Southern, numbered 1405-1409. These engines featured smaller tenders than the second order, but still larger than those of the first order, featuring two-axle bogies and a 12,000-gallon capacity. They also had Walschaerts valve gear instead of the Baker valve gear on previous orders. The final locomotive of the series, No. 1409, featured an extended smokebox with a Coffin feedwater heater.[6][7] This heater was fitted on an experimental basis and was later removed in favor of the Worthington heaters used in the first order.[8]
In 1941, No. 1380 was given bullet-nose streamlining designed by Otto Kuhler for use on the railway's Tennessean service, which operated between Washington, DC, and Monroe, Virginia, connecting in the latter to the Norfolk and Western Railway, who had assigned its streamlined J class engines to its connecting lines.[9][10] When the Tennessean was dieselized, No. 1380 joined the remainder of the Ps-4s assigned elsewhere on other express trains until it retired in 1953, though it retained its streamlining.[11]