Tyrian purple
By the fourth century CE, sumptuary laws in Rome had been tightened so much that only the Roman emperor was permitted to wear Tyrian purple. As a result, 'purple' is sometimes used as a metonym for the office (e.g. the phrase 'donned the purple' means 'became emperor'). The production of Tyrian purple was tightly controlled in the succeeding Byzantine Empire and subsidized by the imperial court, which restricted its use for the colouring of imperial silks. Later (9th century) a child born to a reigning emperor was said to be "porphyrogenitos", "born in the purple".