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The Green Lantern Corps is no longer vulnerable to the color yellow.

This represents a massive change to the core concept of the Green Lanterns, their power rings having always been portrayed as the most powerful weapon in the Universe, with only that highly specific weakness to hold them back. To understand how this happened, we need to go aaall the way back to 1992, in the miniseries The Death of Superman.

As many people know, given the massive hype surrounding the death of one of the most popular heroes in DC Comics, Superman was killed in 1992 by the supervillain Doomsday, who debuted in the miniseries, having appeared from seemingly nowhere. In the aftermath of Superman's death, four new heroes appeared to try to fill his place. One of these was really supervillain Hank Henshaw, also known as Cyborg Superman. Henshaw was working with alien supervillain Mongul to take over the Earth, in the process destroying Green Lantern Hal Jordan's hometown of Coast City, killing everyone who lived there.

Hal Jordan did not take this well, and slid into homicidal insanity, resulting in 1994's Emerald Twilight storyline. Adopting the new moniker of Parallax, he ultimately attempted to destroy the Universe in order to remake a perfect one in its wake. He was, of course, eventually stopped in the Zero Hour miniseries (another of DC's continuity-altering reboots). Hal Jordan spent the next ten years in a sort of purgatory as The Spectre.

After this decade-long gap, 2004's Green Lantern: Rebirth started a chain of events that ultimately resulted in a galaxy-spanning Lantern war. It is revealed that Hal Jordan's Parallax alter-ego was largely the result of manipulation by an actual cosmic entity called Parallax, the personification of fear. Billions of years ago, Parallax had been captured by the Green Lantern Corps founders, the Oans, and imprisoned in the central power battery that gave the Green Lanterns their powers. Since the Green Lantern rings were fueled by willpower, and Parallax represented fear (which erodes a person's willpower and is symbolized by the color yellow), this is the reason the Green Lantern power rings have historically been unable to affect yellow objects. Parallax escaped from the central power battery during Hal Jordan's earlier rampage, freeing him to feast on the galaxy's fear once more but, in the process, removing the "yellow impurity" from the central power battery.

Ever since this miniseries, the Green Lantern Corps has been unaffected by the color yellow.

This symbolic conflict between the Green of willpower and the Yellow of fear became the basis for a future conflict when, in 2006, Hal Jordan's arch-enemy, Sinestro, and the Parallax entity created a new, yellow power battery and began to collect villains from around the galaxy for a new Sinestro Corps to fight the Green Lantern Corps with yellow power rings.

This was, of course, then taken to its logical conclusion as the remaining 5 colors of the so-called "emotional spectrum" began to appear in the DC Universe. Another Hal Jordan enemy, Star Sapphire, is revealed to have been powered by the Violet light of love all along. The full emotional spectrum is:

DC's Blackest Night miniseries, which began in 2009 (and is still, as of this writing, ongoing), details the galaxy-spanning war being fought between not only these seven colored lantern corps, but also an eighth Black Lantern Corps which resurrects the dead to recruit members. It's kind of strange to realize, 17 years later, that this was all kicked off by the Death of Superman story arc.

DC Database Wiki
SomethingAwful.com forums, Blackest Night thread