The color purple is universally associated with power. Roman emperors wore purple, and both kings and the Catholic church wrapped itself in a purple facade. To mock Jesus, the Romans made him wear a purple robe when they arrested him, as Jesus had referred to heaven as "his kingdom".
Why? Not surprisingly, because it was rare. In antiquity, purple dye was obtain by salting a Mediterranean mollusk referred to as murex, and even if the mollusk was free the dye could be difficult to extract. Because of this, Julius Ceasar did the logical thing: forbade anyone except his household wear purple-trimmed togas. And if Caesar did it, then anyone with power should do likewise.
And let him drink from a jeweled cup and sleep on Sarran purple
Virgil, first century B.C.
Source: Kurlansky, Mark. Salt: A World History.
