The white marble temples and statues of ancient Greece, and the grey limestone cathedrals of medieval Europe, did not look like that to contemporaries. In fact, they were painted in a riot of color, which is not exactly to our taste. Perhaps as a consequence, the paint has been allowed to fade and has not been restored. At Amiens Cathedral, however, cleaning efforts in the 1990s revealed what the original colors were; they then figured out a way to project colored light onto the statuary on the façade to indicate how it may have appeared in the thirteenth century (hat tip: Michelle Armstrong-Partida). In a similar fashion, Vinzenz Brinkmann and Ulrike Koch-Brinkmann have spent two decades using “several high-tech methods to uncover the true intended appearances of ancient artwork,” including X-ray fluorescence, infrared spectroscopy, and ultraviolet analysis. This allows them to make digital images of the statues as they may have originally appeared.
Both links will take you to some very interesting illustrations.