Well, the reason you see different color is, Because of the guide coat.
After the actual primer is sprayed on, you take an areosol can of something that looks like flat black paint, and spray it over the fresh primer. The reason for this is.....
When you are blocking the primer to make it flat and smooth, the guide coat shows you where your low spots are, because you sand it off of the high spots of the primer leaving a black low spot. (It just helps you get the finish prefectly smooth.)
Mike