Now, on to the good stuff... the color coat.
For this, you should use a paint that is formulated for your specific application. Example, if you're painting a valve cover, use high heat engine paint; if it's a urethane bumper, use a flexible exterior paint, etc.
Again, make sure that you have cleaned the surface to get any oils off of it, and that the surface is completely dry. You should give the primer coat at least overnight to dry completely. Tack cloth the piece before spraying.
The color should be laid on in several thin coats. Don't worry about covering the the primer in the first couple of coats, just go lightly. In fact, four coats is usually a good beginning for the color. After the primer is completely covered, and the paint has dried for at least six hours, take the piece to the bathtub or sink, and hold the piece under luke-warm water, and sand it very lightly with 2000 grit sandpaper. Let the piece dry, and inspect it to see that the primer isn't showing through anywhere, and that the sanding has created an even, smooth surface. If there are any deep scratches, or any dust blebs, sand them out now. The finished appearance of the sanded paint will be dull and cloudy. Don't worry, that will change when you clear coat it.
Once you have the color coat evenly sanded, and the primer completely covered, let the paint dry over night.