“Shellfish purple was the most highly valued dyestuff in the ancient world because of its exceptionally laborious and hence costly processing, and its unrivalled light-fastness and durability. These characteristics made purple—at least in theory—for many centuries the legally enforced prerogative of the imperial household and government; and throughout the Middle Ages, and even in modern times, it has continued to be an emblem of royalty. But the colour purple remains a mystery, because the early literature suggests not only that it was classed as a type of red, but also that the best purple-red cloth looked dark by reflected light, but a fiery-red by transmitted light, and also had a much-admired surface sheen. . . . The erosion of the surface and modern lighting make this original sheen very hard to appreciate now.”

—John Gage, from Color in Art, 2006.