Medieval tales used skin whitening to signal true belief

Evidence

His skin, that had been black and loathsome, became all white, and was spotless without blemish. And when the sultan saw that sight, well he believed on almighty God… . [He returns to his chamber:] Scarcely did she [his wife] recognize her lord. Then she knew well in her mind that he did not believe at all in Mohammad, for his color was entirely changed. — Sophia Rose Arjana, Muslims in the Western Imagination, 110

Categorization

• QUESTION: The pattern that a convert’s wife recognizes apostasy by color change is striking — is this a folkloric trope with deeper sources worth investigating?