If you don't know what primer is, let us supply an admittedly cursory definition: primer refers to the coat of paint used for 'priming' (preparing) a surface for the final coat. It is often grey, but sometimes brick red (as in the case of Will's and Laura's new vehicle). In case the point of the joke is still not clear, let me elaborate: the truck is so old, and so in need of a new paint job that its current coat is made up almost entirely of primer. Get it? !