The weakness of the titular character in “Green Lantern” is yellow. In the classic Green Lantern mythology, the power rings of the Green Lantern Corps could not directly affect yellow objects or energy. That limitation became one of the character’s most recognizable vulnerabilities during the Silver Age of comics.
Later stories gave the weakness an in-universe explanation. Yellow represents fear in DC’s emotional spectrum, opposing green’s association with willpower. The entity Parallax, a manifestation of fear, was imprisoned within the Central Power Battery, creating the so-called yellow impurity in the Corps’ power.
The rule has changed across different comic-book eras. Experienced Lanterns can sometimes overcome the limitation through exceptional willpower, and modern stories have reduced or removed it in some circumstances. This is why readers may find examples of Green Lanterns affecting yellow objects, but yellow remains the canonical quiz answer and the classic weakness most associated with the character.