I recently had to "opportunity" to swap the motor (an NT855) out of the '68 Pete dump truck that I drive, with a motor out of a '60s
vintage Kenworth (also an NT 855). I suspect that both motors at some time have been changed. But as it relates to this topic, the
motor that came out of the Pete was Cummins tan. The one that we put is had a Cat yellow block, red aftercooler, and tan heads. We
added the auxilary drive from the original motor so it was tan as well. On vintage trucks that may have had the motor changed and/or
rebuilt you can have a little fun with the color, and anything within reason, or that you can document, goes. This probably muddies
the waters further.