[tsimionescu]

I don't think this is right at all. C is a universal constant, and it is not in fact known for sure whether light travels at c or just below (whether photons have a mass). However, individual photons in certain conditions can absolutely move at speeds lower than c. If photons of different energy move at different speeds, this would be detectable by firing the photons at the same far-away mirror, and measuring how much time it takes for each photon to return (on average).