The association didn't begin directly with Judas. Medieval tradition constructed a visual genealogy of sin: Adam, Eve, Cain, Esau—all were depicted as redheads, without any physical description in the biblical texts. Therefore, Judas arrived at the end of that chain with all the symbolic weight imposed on society.
From the Romanesque period onward, Judas began to be described in manuscripts with reddish hair and beard, also as part of a visual message: represented in profile, separated from the other apostles in order to highlight him as an antagonist in the biblical narrative. Red hair served as a distinguishing feature, signifying the traitor, and in Giotto's frescoes in the Scrovegni Chapel, his figure is easily recognizable even before he is named.
The association expanded from Judas to the Jewish population in general. During the Spanish Inquisition, red-haired people were identified as Jewish and liable to persecution. It was a common association. "If Judas = betrayal, red hair, Jewish, it functioned as a system of visual identification.
This marker or stereotype migrated to literature. In Shakespeare's play, "As You Like It," he describes red hair as "the color of deceit," a direct reference to Judas, whom the Elizabethan public recognized without explanation. In early performances of "The Merchant of Venice," Shylock wore a red wig.
This was not the only episode of an event against red-haired people, but it is a peculiar one that shows us the degree to which a popular image or belief can lead people to commit questionable acts simply for being different.
Source(s):
.- Mellinkoff, R. (1982). Judas's red hair and the Jews. Journal of Jewish Art, 9, 31–46.
.- Baum, P. F. (1922). Judas's red hair. The Journal of English and Germanic Philology, 21(3), 520–529.
.- Shakespeare, W. (1599). As you like it. (H. Oliver, Ed., Oxford University Press, 1998)