The color of the primer will to some extent affect the color of your final coat. For instance, if you are painting a tan surface, it will look darker over gray primer than over white primer. So it all depends on the final result you are looking for- this falls under the "pre-shading" category.
If you paint some areas or panels with black primer and others with gray or white, you will end up with a subtle color shading variation after you apply the color coats. The result can be very realistic if done right, but it takes practice.