Before this gets relegated to railfandom, I'll attempt to "re-rail" the topic. I am curious to know what/when the last Green and Gold Steam locomotive operated in regular service on the Southern. I'd assume it was a PS-4 that was relegated to a local of some sorts.

My guess would be one of the PS2s on the Louisville-St. Louis train. In fact, the latest Classic Trains said that too, that they ran until a couple days before SR's "Last run" photo was taken.
They were Green engines.

I hope that she's being painted. That means she'll be out on the main line that much sooner!

There was an article on the Southern in Classic Trains? Will definitely have to check that out. Anyways, Frisco 1522 is correct. According to the Ties May-June issue from 2003, the last steam passenger runs occurred on Sunday the 14th of June, 1953. Both passenger runs were trains 23 and 24 between Louisville, Ky and Princeton, IN on the St. Louis-Louisville division. They were powered by P-2 Pacifics 1231 and 1317. At the time, The CNO&TP had been dieselized for a month. The idea of bringing 6330 out of storage to run one final steam run was the brain child of Bill Brosnan. He conceived that it would be a better public image if the Southern dieselized in the heart of its system instead of in Indiana.
So to bring all of this back to Tim Reynolds question, 1231 and 1317 would have been the last Southern green and gold steam power to run.