"A woman that hath red hair, or freckles on her face cannot be a good nurse."

I've found another example of the belief that breast milk from red-haired women is unwholesome today. This one comes from a translation of the works of Ambroise Paré, printed 1649.

Why a woman that hath red hair, or freckles on her face cannot bee a good nurs.

[The milk] ought to bee sweet, fragrant, and pleasant in smell; for if it strike into the nostrils with a certain sharpness, as for the most part the milk of women that have red hair and little freckles on their faces doth, it prognosticates a hot and cholerick nature: if with a certain sowerness, it portendeth a cold and melancholick nature.