Technically, I beleive the underbelly "body" was painted with Slop grey (mix of used/not used extra paints adds up to grey). With Cosmoline added, this all becomes very black in appearance over the years. Frankly, I'd keep it all there if there are any plans at all to drive the car. It is a terrific base if the car is clean and mostly rust-free.
BUT, it is totally accepted in the car-show and sale world to use red primer. Red Oxide primer is great for hitting on the underbelly a few times. It can be painted over layers that you bungle/ding/scratch and many layers go on very easily. Having a lift to walk under and touch up areas makes it a real treat.
I painted the fender wells Black and then hit with undercoating spray, but many folks like to paint the inner fenders "body color" and then hit with with undercoating, especially the fronts of the front fenders and the splash areas. It gives you a lot of forgiveness with imperfections under there.
Tunnel sprayed black gives a great appearance and detail. Also, check out my pics of the junction areas in the front suspension/underbelly where you go from primered/slop grey to black in the front..especially at the lower A arm/shock tower bottom junction. There is a transition where you kind of "mix" the red and the black.
Another area for careful detail is the lower body pinch weld area. There is black out on the outer lower body adjacent to the pinch weld and the pinch weld itself. Flat Black paint. I didn't show this detail in the photobucket spread, unfortunately. There is an especially good amount of paint here in the rear panel, on the pinch area and the horizontal outer body area. Front fender lower body can get a touch too, but it tends to show if you're not careful. Use a long piece of cardboard for a guide and hold it along the line of body paint that is to remain colored and clean and spray-away.
The overspray on the underbelly is technically acheived when you're painter hits the lower body from a perfect angle with a good spray gun. What doesn't hit the lower body/pinch weld area hits your underbelly, and in a good booth it's gonna go all over the underside. If you're working the underside last, though, and detailing things under there, it's tough to keep the overspray look and then do your work, especially if you're touching up with red oxide primer last (what I did"..so you can get a rattle can and get the look if you can find a good match (silver is awfully easy!). Again, Cardboard sheild and spray away. It's a three minute job and adds a cool "touch" that everyone thinks makes it all look so concours
Other little things that go a long way:
The correct Red and silver and grey on your chassis/suspension bolts (AMK products or WCCC)
The correct color on your leaf spring "ties" (those clamps that go around the leafs)-these are more bright steel color, the springs are more cast-grey)
Correct aluminum paint on the tranny and stamped steel paint on the tranny oil pan
Correct grey vs. Stampted steel paint on hand brake cable attachments and brackets
Correct low gloss black on brake splash sheilds
Correct Low gloss black on Brake backing plates, axle, and rear end
Correct Shiny Red Oxide (hit with clear after oxide) on front of rear end's pumpkin
Correct Bare metal cast color paint on suspension and steering components up front. Or use bare metal all cleaned and then oil or clearcoat (I had some painted and some just wire wheeled/cleaned and oiled up good
Oil up your leaf spriings and rear brackets and ubolts for protection. Even if they're painted, they will rust. And keep your rubber all treated with WD 40 for brightness and junctions between paint and rubber nice and clean and sharp.
Sheesh it goes on and on...just make it a driver and don't worry abou the underbelly

Did I mention that factory Cosmoline is great stuff!!??