Primer serves basically two functions. One, it provides a "tooth" for the finish coats to stick to, thus creating better paint adhesion. Two, it allows you to better see small imperfections in your construction, allowing you to correct them before you apply the finishing paint. Primer also seals areas where you have applied putty and blends them into the rest of the model. So, if you are going to use primer, do it after you have finished construction. Most primers are light gray, which would work fine. They can also be dark red or black. Most folks wash their models after construction, since skin oils, fingerprints, nacho cheese and other foreign objects could be attached to the surface.