Betwixt vibrant red and calm blue sits ‘royal’ purple. You may call it mauve or violet, and it may not always be accompanied by a catchy preliminary adjective, but the colour purple was the most sought after in history, and has roots in mythology, piety, art and royalty. The most refracted colour when light passes through a prism, purple is at the far end of the visible colour spectrum, and is the hardest colour for...