I think the crud your referring to is the buildup that occurs under the body. Oil leaks + dirt etc. I think Jim has it right about the wheel wells. I will find my LH side front wheel well and see what it looks like. I confirmed the rear pans were sprayed.

I have painted these cars back in the early 80's and current (2017). Pontiac primer was black, red or grey. It must be covered with top coat or your going to have trouble. The primer is porous and cannot shield itself from the UV rays. Exposed primer will allow rust to start again.
Primer sticks to bare metal, bondo and topcoat. Top coat sticks to primer only. In today's world it's typically bare metal, primer sealer, bondo, primer filler, primer sealer, top coat, clear coat.

From what I have read and experienced (actual paint removal experience), I would argue the car body was dipped in primer(s), coated with either black or grey primer, and to a lesser extent (in 68') red oxide. Then most of the car was painted with one coat of top coat. The top coat was a lacquer based paint. If you ever sprayed this stuff you would know that it goes on very dry and dull. Paint over spray turns into dust very quickly.
It will not stick to the shop floor. So if there is "over spray" on the body it won't wonder too far. The painting of the top coat over the primer was indeed
deliberate and not drifting wet paint vapor failing onto the paint surface. Some of you may feel it's over spray because of the quality of the paint. GM painted multiple coats on their cars + heavy buffing to get a shine. They did not do multiple coats and buffing of the body shell areas we are focusing on.
NO GM car (top side or bottom) came out of the plant looking as good as 4dabird's floor pan!

P.S. One of the best places to confirm top coat color is under the rear axle bumper on the rear frame. The top coat was sprayed on the under body before the axle bumper was installed and the paint was sealed from any salt, water etc. so it indeed is a good time capsule.