Brewer's: City of the Violet Crown
Athens is so called by Aristophanes iostefauoz (see Equites, 1323 and 1329; and Acharnians, 637). Macaulay refers to Athens as the “violet-crowned city.” Ion (a violet) was a representative king of Athens, whose four sons gave names to the four Athenian classes; and Greece, in Asia Minor, was called Ionia. Athens was the city of “Ion crowned its king” or “of the Violet crowned.” Similarly Paris is the “city of lilies” —i.e. fleurs-de-luce or Louis-flowers.
I do not think that Athens was called from “the purple hue which Hymettus assumed in the evening sky.”