Which color could only be worn by a Roman Emperor?

The color that could only be worn by a Roman Emperor was purple. Specifically, only a Roman Emperor was allowed to wear the 'trabea' which was a toga entirely colored in purple. It was always worn on ceremonial occasions. Statues of gods were also dressed in the purple trabea toga, emphasising the importance of the color and of course associating the Emperor with the Roman gods.

The Roman custom of wearing purple togas, according to popular historians, is said to have come from the Etruscans. An Etruscan tomb painting from the 4th century BC clearly shows a great nobleman wearing a deep purple and embroidered toga. In Ancient Rome, the 'toga praetexta'as an ordinary white toga with a broad purple stripe on its border. This item was worn by both sexes, the citizen-military, certain magistrates, and appropriate priests.