Tom, that thread you shared brought back memories! Below are pics of the MTH RK L1 project that prompted my original question on PRR steam lettering in that thread. Note the yellow / buff lettering on top of Pennsy Dark Green Locomotive Enamel (DGLE).
Pat, Pennsy DGLE is the color you want for a Pennsy steamer. I use Badger Modelflex acrylics in a four parts Steam Power Black, one part Brunswick Green, and one part Weathered Black mix to create my version of DGLE. The result is a very dark green that looks black. (Modelflex "Brunswick Green" paint is much too green by itself to represent Pennsy DGLE.) It looks true to me and is almost an exact match for MTH factory painted Pennsy steam. The difference between DGLE and black is noticeable in a side-by-side comparison. I'll try to post a picture tomorrow to illustrate the difference.