Yellow is prototype for the PRR (Note Amtrak has upgraded the NEC signals with green, yellow, and red bulbs see my post in the NEC thread). PRR signal aspects had only yellow light positions.
On the right side (of the cab signal box) are signals for normal (searchlight, Safetran, etc) signals, so the locomotive can run over any cab signaled territory, on ex PRR tracks, modern day UP, CSX, or any other route that needs them. Also for reference not all P42DC's have cab signals. Not sure the number series on which don't.
EDIT: They are yellow on the cab signal box, not white that I had mentioned above, but since deleted for clarity):
http://www.railpictures.net/viewphoto.p ... 16&nseq=10This cab signal box is good for the RF&P (CSX), Conrail (PRR), Amtrak (NEC), I.C.G., and UP:

Whatever the route the locomotive is running the conductor would turn the nob to the railroad and lock it in place with a pin.
Last edited by Chessie8638 on Tue Jul 05, 2011 12:34 pm, edited 5 times in total.