The important thing to remember about priming is that it provides a surface for the paint to key onto, so it's not about laying down loads of colour. You can prime white and spray black over the top, or just go with a straight black. Two thin layers should see you sorted ready for your base colour(s) to go down. This is when you start to define the ultimate colour of the miniature. That said, it is essential to prime with black if you want a dark palette or white if you want a bright palette. Consider priming with grey to get the happy medium between the two.
If you're feeling really fancy you can spray black as the initial prime, with a dusting of white at a 45 degree angle to create a Zenithal lighting effect. This will shine through the colours, assuming you keep the layers nice and thin creating an instant lighting effect.
