Well...not ALL the SP GS-class locomotives were colorful. Classes GS-1 and GS-6 (as well as GS-7 and GS-8's, but the -7's were ex-SSW locomotives, while the -8's were ex-SSW, ex-SP locos(originally GS-1's)) were always black. And Daylight locomotives that were assigned to the Portland Division were immediately painted black (because after the first run, the WERE black!)
But those are the exceptions to the general rule... remember the Mt's that received the Daylight treatment in the early 50's for service on the San Jacquin line.
But you're right...there were a LOT of steam locomotives painted in colors!
But we remember the vast majority who toiled their entire lives in basic black.
Bill Daniels
LaLaLand