Rick-
Those locos aren't actually black. They're Dark Green Locomotive Enamel, also known as "DGLE" or "Brunswick Green". Compare with the black NYC locomotives on BLW's site:
http://www.blwnscale.com/ll-c-liner.htmThe DGLE single-stripe scheme on those units was a freight scheme. In 1955, the PRR did go to a single broad stripe with a large keystone for passenger units, but that single pinstripe you referred to was for freight. The original diesel passenger scheme also used DGLE, but had five "cat whisker" pinstripes, originally applied to the GG1. In 1952, the PRR started transitioning to tuscan red for passenger units.
If you don't already know about it, Keystone Crossings (
http://pennsyrr.com) is a great resource for research on all things Pennsy!