Creating terrain overlays

[Planner1]

I have a terrain shape that I'd like to color based on overlays very much like in Simcity where you might display "fire protection coverage", "population density", or "high traffic areas". Does anyone know how this can be done in Xith? I initially thought to color vertices in the terrain shape geometry but I realized this would not work for culled terrain data because some polygons may be too big. I'm not familiar with the uses of the stencil buffer in this regard but it seems like a possibility. Any ideas, especially in terms of Xith's usefulness for this?