Ok, I have to ask a stupid question. What color would be most appropriate for the under color of a modern tank. Like when the sand paint is chipped off, what color would be showing through. I am sure it would be different for US, British, Soviet, Chinese, etc.

Any help would be appreciated.

Modern US vehicles are primed in a red oxide. However, since CARC paint is an epoxy paint and very tough, and since they get repainted every couple of years, modern US vehicles rarely chip down to primer. If anything, they will usually only chip to an earlier layer of paint.