[vtkusers] How to color for a vtkQuad-front different than its back?

[Jens]

Hi,

I would like to visualise boundaries in 3D. For that purpose I use vtkQuads.
Now I would like to color these vtkQuads that way, that the user can see
if he is looking at the front or at the back of the quad.

Does anyone knows how to color the front of a quad in a different color
than the back?

Greetings
Jens