I have heard that redheads were thought to be vampires, or Judas's descendants, good for spell ingredients, etc...
What were common myths these about redheads in the medieval period of Europe (300 AD - 1600 AD)?
Red hair was a sign of witchcraft in Christian Europe and it's said that it was often seen as a marker of guilt in the eyes of witch finders.
There are many examples of redheads being stereotyped as untrustworthy in medieval times. The "Proverbs of Alfred" warn not to choose a red-haired person as a friend and the "Secretum Secretorum" warns against using redheads as advisors.
Kemble notes that similar warnings against short,
tall, and red men occur in German and medieval Latin. He observes : —
' The faithlessness of red-haired men is known to have been a widely
prevailing belief, and to have passed into the proverbs of many European
countries : Judas, ' in the painted cloth,' has red hair, allusions to
which in the works of all our old dramatic writers are far too numerous
to require specific reference.' There is, however, only one such reference
in Shakespeare; in As You Like It, iii. 4. 9. See D. ii. 264, 265.
The author Ruth Mellinkoff, in her excellent book, "Outcasts," commented upon this prejudice, believing it to be a product of red hair's minority status in society.
Red hair, a red beard, and ruddy skin - separately or combined - have been considered suspect, impure, and dangerous because they did not meet the standards of the normal. That is essential to keep in mind is that they are minority features in all racial and ethnic groups, even among the Irish, who are widely thought to have more redheads.
She also relates an early example of Christian condemnation of red hair, quoting from a letter from St. Jerome which gave advice on raising daughters.
Do not dye her hair red and thereby presage for her the fires of hell.
The medieval mind also attributed magical properties to the bodily fluids of redheads. For example, it was believed that the fat from a red-haired man could be used to make poison. Likewise, Theophilus Presbyter stated that the blood of a red-haired man was needed to turn copper into gold. The urine from red-haired boys was also used to make the paint for stained glass windows.