Keep in mind that these cars are over 40 years old now, so the fact that there is grease covering things up doesn't mean it hasn't been messed with during that (almost) half century of time.
Pontiac never painted the TH350 any color - these transmissions were always left bare cast aluminum. If the tranny has been painted, somebody has done a little creative work sometime during the car's life. Some tranny builders do this to identify the transmission as their work - it keeps people from trying to get warranty repairs on a transmission not repaired by the builder. When I was building automatics, I painted the pans GM coporate blue to I could easliy spot one of my transmissions: Since I actually did warranty rebuilds on near-new cars back in '77, there are probably people who think there were "original" TH350 and 400s with blue pans...
Former instructor - GM Training Center
TH350, TH400, Q-Jet Carbs & Ignitions Systems