Okay. When Hal Jordan was chosen to be a Green Lantern, he was only picked 'cause he was closer to Abin Sur than Guy. Eventually, Guy did replace Hal for a while, but a faulty power battery put him in a coma. Fast forward to the Crisis on Infinite Earths. Guy was revived buy a splinter group of the Guardians, and was given a Green Lantern ring, with which he was supposed to lead an attack. He was stopped by his fellow heroes, however, because if he had succeeded, it would have been a catastrophe for the heroes. Later, the main power battery was blown up, and only Hal's ring, Guy's ring, and a few other GL rings survived. Guy replaced Hal eventually, and became a member of the new Justice League. He was finally put in a challenge where he and Hal would have to battle for the title of Earth's Green Lantern, and the loser would lose his ring. Guy lost, and so the Yellow Days began!