I questioned Galen on this subject at Carlise this year and the only paint underneath is primer and overspray. Here's a picture of mine that I scrapped undercoating off of. My car is FE5 Ralley Red and if you look at the picture, you can see that it does in fact transition from red paint to redish/brown primer. That had me fooled into thinking the whole thing was painted body color underneath and it isn't.
However, I did powder coat all the suspension and did pain the underside body color, and I drive the car (1500 a year max) and the body color underneath really has been pretty good but I do have a lift and can keep it clean underneath too.
So, I don't think black is right unless you have a black car and it's only the overspray that would be right unless of course, you have black undercoating.