Quote - Hello,
even if the is the Poser specific forum, I would like to add that it is software dependent.
Black is neutral for Poser, grey (128,128,128) is neutral for Carrara (and most likely DS as well).
Other software may use even different settings. 

I was going to say, 50% gray in Poser is NOT neutral. It raises the surface a fraction of an inch, which is quite noticeable where one displaced body part ends and a non-displaced one begins. It also can cause render poke-throughs in snug clothing even if you don't see it in preview.

So, if black is neutral and white raises the surface in Poser, how does one create a a displacement depression? There is no blacker than black, is there?

I would use displacement for facial ornaments, etc., where expressions will not carry a prop piercing with it. (Because the nose, chin, cheeks are not separate body parts and the piercing is parented only to the main head).

Displacement is quite effective in this situation. Put the facial ornaments on a layer above the facial texture in Photoshop or the equivalent, Save As a jpg (which merges the layers in the copy). Then make displacement map with the ornaments in white. In the Mat Room, make sure Use Displacement is checked on the face material zone, and play around with the dial to get the right amount of "raise" off the surface. A very small amount is usually best. (Again, you have to render to see the effect).