Why was purple considered the royal color? The answer lies in economics not in aesthetics. Purple is rare in nature. A Toga’s worth of Tyrian purple die, about 1.5 grams, required the beating, drying and extracting of mucus from the hypobranchial gland of some twelve thousand Murex mollusks.
Legend credits its discovery to Herakles, or rather to his dog, whose mouth was stained purple from chewing on snails along the Levantine coast. King Phoenix received a purple-dyed robe from Herakles and decreed the rulers of Phoenicia should wear this color as a royal symbol.
The practice was later adopted by the Romans; to wear purple, therefore, was to show off your great wealth. Purple is an interesting example of a snob or Veblen good because it is clear that if purple had not been expensive it would not have been greatly desired. Indeed, do we see any great demand for purple today? If purple paint were say 25 or 50% more expensive then people would switch to substitutes but make it 500 or 1000 times as expensive and it becomes a fashion statement.