Vrml file properties for Color Printing

Maybe not the best place to ask but I do all my shopping here. I’m looking to try some color printing and there are several processes available. Shapeway’s sandstone, to me, is the least attractive because it’s grainy and the color resides only on the surface. The other process like Polyjet and i.materialize Multcolor are UV cured and give color throughout the piece. My confusion is that all require a vrml file, which contains the color/texture information.

My questions are:

  1. If the printer can print color throughout a piece, how does it know what color to use in the interior, since if I assign vertex colors to a mesh, it’s only on the surface?
  2. Do I have any idea what I’m talking about?