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).
All 4 steps done with an airbrush: Prime Vallejo Black, then Zenithal Vallejo Grey, then one spray directly from above with Army painted white (any white should do). Finally, I just sprayed Skeleton Horde until it started pooling, at which point I stopped to avoid over spraying.