There is possibly a play on words between the Hebrew karmel and karmil (the colour purple or crimson), referring to her hair that is possibly dyed purple. This dye was very expensive and in early times was associated with royalty.1 Her hair may not literally be dyed purple. It may simply be a way of expressing the richness of its colour and beauty.
5 Your head crowns you like Carmel, and your flowing locks are like purple; a king is held captive in the tresses.