Purple's association with royalty and status traces back to the extreme cost of producing Tyrian purple dye in antiquity — an economic fact, not an inherent property of the colour, that carried forward into later royal and religious traditions.
Tyrian purple dye was produced in antiquity from the mucus of Murex sea snails; thousands of snails were needed for a small amount of dye, making it enormously expensive and a marker of status across the ancient Mediterranean.
Mediterranean · antiquity
Roman and later Byzantine emperors restricted the wearing of Tyrian purple to imperial rank, reinforcing its association with supreme authority for centuries after its original cost had already made it rare.
Rome and Byzantium · Roman Empire to Byzantine Empire
The association between purple and royalty persisted in European heraldry and court dress long after synthetic purple dyes made the colour widely affordable, illustrating how a historical scarcity association can outlive the scarcity itself.
Europe · medieval period to 19th century
Every claim above is scoped by the specific culture, region and/or period it applies to — none of these associations are presented as universal. See pigment and dye history for more.