Purple was an exceptionally hard pigment to make throughout history. The Romans made purple pigment from a specific type of sea slug that was needed in large quantities. It became synonymous with wealth because of the labor intensive and expensive extraction methods. Emperors later wore it to show their godly status. I guess Miffy pictures itself as immortal royalty in the same way, though the Cheshire Cat is also purple and has big yellow eyes and otherworldly abilities just like Miffy, so maybe there’s a connection?