Here's what happens in Bryce. If a triangulate the 2 color object Bryce reads the geometry OK but only puts the first color it finds in the .MTL on the object. As I said above I was real surpised that your 1 color object loaded correctly. What I would do in FormZ would be to Seperate (or Split) each of the different colored facee into a seperate object for each color. This works well cause you can give each of the objects different Material Editor parameters.