Once upon a time, before the DC Universe suffered through the so-called Crisis on Infinite Earths, the Man of Steel had a very colorful past, and few things were more colorful than kryptonite, which came in a variety of hues. Beyond the well-known green version, which was capable of weakening and eventually killing Kryptonians, there was red kryptonite, which caused weird (but temporary) physical and psychological transformations; white kryptonite, which killed any plant life within its proximity; blue kryptonite, which was only harmful to Bizarro and his ilk; and so forth. Gold kryptonite, however, could remove Superman's powers permanently, which is exactly what it did at the end of Alan Moore's famous "final" Superman story, "Whatever Happened to the Man of Tomorrow?"
In the epic tale, Superman endures a series of unusually dark and twisted battles with members of his rogues' gallery before realizing that the entire thing had been orchestrated by Mr. Mxyzptlk. After the imp emerges to take responsibility, Superman manages to trick him when he attempts to return to the Fifth Dimension and kills him. Feeling the weight of Mxyzptlk's death on his shoulders, he declares, "Nobody has the right to kill. Not Mxyzptlk, not you, not Superman. Especially not Superman." With these words, Superman silently walks into the vault within his Fortress of Solitude where he stores his gold kryptonite and removes his powers himself.