Well ... it looks like what I'm describing is already in place when you turn shadows on :
The color of the face is not determined by the angle between the camera and the perpendicular to the face, but the angle with respect to the sun. You can tilt your view, the faces don't change color.
Still, I'd be interested in coloring the faces in "false color" based on the sun angle.
Anybody would know where to start?
Thanks.
Paul