This article accompanies the fable
The branding, the symbol of shame, which resembles the anti-Semitism and homophobia of the Nazis, the Scarlet Letter of Hawthorne's adultress... Former and repentant Cathars and even those related to them were forced by the Inquisition to wear the Yellow Cross to acknowledge their heresy, for the Cathars never worshipped the cross.
Such discriminatory dress codes emanated from Baghdad and the Islamic world in the 700's and 800's and clearly yellow was the favored color of discrimination. In the decades after 1215, much of Europe adopted some form of color coding and red was often used too.
The Cathars' contemporaries, the Knights Templar, had different crosses: red for the blood of Christ. They were a monastic order and took vows similar to the Cathar perfecti: chastity, piety and so on. They ended up much the same way as the Cathars: exterminated amid allegations of sodomy and Satanism, victims of their own preference for secret rituals.