Cars these days are painted with a basecoat/clearcoat system. The base coat is simply the color coat, and then it's topped with clear to get the glossy finish. Back in the old days they didn't use the 2-step process, they just went with a paint that was glossy to begin with. So when the auto parts store guy talks about a "basecoat," what he means is the actual colored paint that gets sprayed on first, the "base"...before the clear goes on.