The Roman emperors were batty about purple, forbidding their citizens from wearing it under penalty of death. Ptolemy of Mauretania visited Caligula draped in a majestic purple cloak, which Caligula interpreted as an act of war, and so learned this the hard way. Centuries later, Henry Howard, the Earl of Surrey, made the same sartorial miscalculation. Among the evidence presented against him on trial for high treason against Henry VIII was that he had been seen wearing purple. He was beheaded. Like most conceptions of value, purple’s high stock was mostly about scarcity. Unlike greens or yellows or pinks, purples...