I thought, incorrectly apparently, that some were newer motors while others were older runs. I know that the color of the original motor in an M939A2 series, for example, denotes the rough year of the truck. In that case, at some time in 1991, they started using Cummins tan. Before that, they used black. Both black and tan were new motors, not rebuilt. As I said, with the M35A3s, I thought it must have been the same. However, I was corrected and told that both colors came out side by side. I owned an M35A3 with a yellow motor. I like the yellow motor better because it LOOKS like a CAT motor. With that said, I would not care what color the motor was.