I was looking for a way to speed up my painting and decided to try airbrushing contrast. As you can see, the result is fairly decent considering the time spent. What you see here is a zenithal priming (black, grey then white) followed by airbrushed skeleton horde (with about 20% thinner in it).
On a recent project, I simply primed white, and some hard to reach parts of the model remained white, which I really disliked, because I spent a lot of time teaching these unpainted parts.
Zenithal makes the model white, while still leaving the hard to reach parts black, which registers to the eye as shadow and gives, in my opinion, a better result with less efforts. Contrast also makes the transition between white and black (light and shadow) really smooth and natural, even on brighter colours.