There are specific paints that are used as primers, usually in white/grey/black
The color of the primer depends on what colors you're going to be using. I forget what primer goes best for dark/light.
Your best bet is to use grey paint as a base coat, either being a specified primer or just normal paint.
You don't have to put on a base coat/primer, but it helps the colors be a little brighter and the paints will actually stay on longer.
I also suggest using a sealant of some sort, just to preserve your work after it's finished.