Red Light

Superman draws his power from Earth’s young sun – because of the Sun’s light, the Man of Steel can be considered invincible. However, when red light hits him, he becomes weak and has the power of, well, a normal human being.

Doctor Light is just one of the villains that could defeat the Man of Steel this way. In a way, Doctor Light (who can manipulate all ambient light) could drain all the powers from Superman, or could turn the yellow light from his body into red light, thus weakening and eventually killing him.