You may also want to consider using all three colors all the time. I.e., one can normally see the color of the non-lit lights. Then increase the brightness of the "lit" light either by using a brighter texture, or adding (and saturating) a white color to the output in the pixel shader based on a flag of some sort.
EDIT: Any reason to use textures? Why not just set the rendering color for each "light lens" portion of the mesh?
sorry Buckeye can you elaborate on that idea