The 69 famous image shows a black under body. You can see red on the rear frame rails and where the rockers were painted but the underside is black.
Take a look at this closeup. Your probably right Harold.
The amount of body color on the undercarriage is said to vary significantly. Several years ago on the Firebird List, one of the members posted an interview with one of the paint line ex-employees. I lost the saved post on an old AOL account long ago. I recall that after the manual part of the body painting some supervisors on the line encouraged the painters to hit more of the undercarriage than others. I also believe that as a color on the line was being switched over, the painters would try to run out their sprayers on the undercarriage. My '68 had a significant amount of body color except in the tunnel/hump and other higher recesses.
