Quote - Is it possible to have a shader react to light and Shadow? I was thinking if it were possible to let the brick shader inverse its colors where the shadow hits it. Like in Sin City. Is it possible?

Surely. I don't have Poser on my office PC, but I think this should work:-

Use a Blender node to blend between the original wall texture and an inverted version. Set Blend value to 1.0, and plug in a threshold 'switch' which comes from a Diffuse node fed via a Gain node with high Gain, to give high contrast between diffuse illumination above and below the mid-point.

The threshold in this setup would be a diffuse illumination value of mid-grey (0.5). You can adjust the threshold if necessary by adding to or subtracting from the Diffuse node output before input to the Gain node.

Izi

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)