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Road paint has tiny glass beads embedded in it (or at least it used to), to provide the necessary reflectivity. I think the paint on road signs has something similar. I guess one could have three layers of beads: opaque black, opaque white, and transparent, and for the "white" cells float the transparent beads above a middle layer of white beads, with the black beads at the bottom, then just move the black beads to the top for "black" cells. I don't know how well this would work in practice, even for a sign -- it seems to me that three layers would be quite a bit harder to manage than two. Or maybe the "white" beads in the current two-color system could be mirror-coated somehow. But given the substrate fragility problems reported for the iLiad, at least, I'd be skeptical about putting this stuff on a road.
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