In the ancient Greek theory of medicine, which long governed medical treatment, it was believed that an imbalance of certain "humors" produced one's physical symptoms. Jealousy was considered to result in an excess of bile, which would give a pale-greenish cast to the skin. This idea remained popular for centuries, though it underwent some transformations. It famously appears in "Othello," when Iago refers to jealousy as a "green-eyed" monster.
Lanfranco has blue eyes, but she just warned me while we were discussing this question that they could turn green in a hurry.
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