I think he meant the underside of the panels. On my 67 they are black so the parts must have been primed black overall before body color was sprayed.
This was taken from Jhn Z's article at:
http://www.camaros.org/assemblyprocess.shtmlPaint shop: Takes the raw front end sheet metal panels (hood, fenders, header panel, front valance, inner fenders, radiator support) from the stamping plants, welds the fender reinforcements to the fender skins, and cleans, phosphates, primes, seals, and topcoats the outer panels; cleans, phosphates and dip-primes the inner fenders and radiator support. After painting, all panels are conveyed to the Sheet Metal Line. Also has a low-temp paint system for grilles, consoles, steering columns, ashtray and glove box doors for delivery to the Trim Line, and a wheel system that primes and paints wheels and conveys them to the wheel & tire assembly area.