The center speaker grills were also molded in different colors. However, mine has evidence of paint on the underside. When you scratch it, it's blue underneath. Dad bought the car new so I know 100% that this is the grill the car was born with. Weird.
The main reason I had any doubt about the factory painting plastic versus it being done later by someone else is the paint job was pretty high quality. It bonded well and looked good. No runs or sags and all of the texture of the plastic was still very well defined. I wouldn't expect that from most backyard paint jobs on parts that would take a considerable amount of prep work and skill to do a convincing job where the plastic didn't look painted. I would guess certain kinds of paint are one key to a good looking job.