Parts would have been stamped and painted red oxide (sealer) at the plants were they were made. The primer protected them in transit and (if shipped to the dealerships) as service replacement parts/sitting in storage
Light gray on some panels would have been a primer surfacer coat to fill small imperfections applied on the car assembly line
The dip was not used on the cheaper car lines in 67 that was saved for the more expensive cars that Cougar, Falcons and Mustangs