The Sith are the great villains of the Star Wars universe. While they come in all shapes, sizes, and species, a few traits distinguish them from everyone else. Among those characteristics are an apparent love for dark clothing and red lightsabers. However, from a physical standpoint, the only real commonality between the Sith is the manifestation of yellow eyes.
Source：https://www.cbr.com/sith-lords-yellow-eyes-star-wars/
