Did you set EmissiveMaterial.Brush to Brushes.Something? Those brushes are frozen. Clone it, modify it, and set it back and you'll be good (or create the Brush by hand instead of using the Brushes class).
If something is white, you won't notice the EmissiveMaterial because it's an additive blend and white + anything = white. DiffuseMaterials are a source over blend (a * S + (1 - a) * D) so you could add in another DiffuseMaterial to subtract away from the first. But I would try using the color knob properties because then you'll avoid multiple Materials. For example, if using DiffuseMaterial, set Color to (1.0, 1.0, 0.5, 0.5) and the surface will reflect twice as much red light as green and blue. That'll handle the white case too.
