Primer polygamist
I usually use white primer, but I admit I vary depending on the subject. I have been using black prime on my Sisters of Battle. Not only is this the choice of my mentor, Dragonsreach, but it seams fit the dark, somber mood of the pieces.
For greenskins - dragons, lizardmen, orcs and the like - I use gray primer. Green on white looks cartoony, and green on black is too dull. Gray seems to be just right.
But you guys are talking about priming, not underpainting. Underpainting is a technique where you use a different color on top of your primer, under the paint, to create an effect. It\'s usually use for lighting.
I haven\'t done this myself, but I have taken black primed minis and drybrushed them with white. Why? Makes it easy to pick out details, and you have a really good idea how your shades and highlights should flow.