Depending on your source, there either was or was not a paint color called "Brunswick Green" for the Pennsylvania Railroad. There was, however, an official color called "Dark Green Locomotive Enamel" (DGLE), which is the color applied to all freight locomotives. Essentially, it was black with just a tinge of green (depending on how the sunlight hit it, etc.). Take a 55-gallon drum of black, add 1 quart of green - Voila!
Ex-NYNH&H SS Opr