The GM plant in Arlington, TX didn't paint them at all...at least not that I ever saw. In fact, a painted transmission was a dead giveaway it had been rebuilt. Different rebuilding shops around here used different colors so they'd know what they had done in case somebody came in with a problem and they had unique colors of paint. There were a few shades of blue, some used gray, none used black (too common and easy to mimic), one used sort of an orange/yellow...stuff like that.
