Royal purple
Purple as the colour of kings and emperors, a link that comes from the cost of ancient purple dye.
Also: imperial purple, born to the purple
Purple became the colour of rulers in the Mediterranean world because the dye, made from sea snails, was so costly that only the richest could afford it. Roman emperors wore it, and children of Byzantine emperors were said to be born to the purple, after a palace room lined with purple stone. The link carried on in European coronation robes. It is not universal. In China the imperial colour was yellow, and in parts of West Africa and South-East Asia other colours and materials marked royalty. Cheap synthetic purples arrived only in the 1850s.