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.
The Cathars' Yellow Cross derives from the Yellow Badge that the Jews were forced to wear. Sex is at the root of all this evil since - horrors - it was becoming impossible in some regions to tell what religion somebody else belonged to. The Fourth Council of the Lateran of 1215 famously declared: "At times it has occurred that Christians have had sexual intercourse in error with Jewish or Saracen women and Jews or Saracens with Christian women. That the crime of such a sinful mixture shall no longer find evasion or cover under the pretext of error," so Jews and Muslims must be distinguishable by more than just their clothing. The Yellow Badge was actually a yellow or white circle worn on clothing and clearly visible. Many got away with wearing distinctive hats or robes instead and codes varied by country.
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.