Vrml file properties for Color Printing

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.materialise Multcolor are UV cured and appear to 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 and can vary from point to point?
  2. Do I have any idea what I’m talking about? I’d appreciate any advice or tips.