I had a '70 Road Runner and the entire underneath was nicely painted. The car was not restored (this was 1975) and it only had 27,000 miles on it. Like I said before, there really is no "factory correct" when it comes to this subject.
Primer color varied too. I heard that a lot of leftover "paint" made its way into that vat of primer, so while basically grey, it could be tinted too. GM cars exhibit this variation too.