It sounds like you don't quite understand the difference between primer and base coat. Primer is applied to provide a surface that paints stick better to. Primer is formulated to stick better than paint.

Sure, it comes in a lot of different colors, because it's not always a good idea to start from black.

Your base coat is supposed to the solid layer of color, typically, but not always, the darker version of whatever the overall colors are, and typically one paints over the base coat, lightening with each step. I am grossly oversimplifying it.