The Colors Out of Space, Part 1

Thomas Wolfe called one of his novels You Can’t Go Home Again. Well, Superman has gone Wolfe one better. Not only can Superman not go home again, pieces of home keep following him around, trying to kill him!
As Superman tells us in 1959's "The Curse of Kryptonite" (Superman #130), "Kryptonite is my Achilles Heel … the only substance in the universe that can harm me! It was originally formed years ago when the planet Krypton, the world on which I was born, blew up! A nuclear chain reaction converted every chunk of the exploding world into Green Kryptonite!"
As we will later see, different swarms of Green K meteors passed through different-colored space clouds, creating other forms of Kryptonite, although the lettercol of Superman #243 assures us that "they are all isotopes of the same element."
But before getting down to those specific types of Kryptonite, here's a general rundown of the element, as gathered by yours truly from a few hundred issues of Superman, Action, Superboy, and a few other titles.
Kryptonite was drawn to Earth in the wake of the space-warp drive created by Jor-El for baby Kal-El's rocket (Action #500). It has strong radioactive characteristics, and this fact has from time to time inspired scientists to attempts Green K-powered engines, as in Action #224, Superboy #65, and World's Finest #76 -- not to mention the Green K-powered atomic heart of John Corben, Metallo (Action #252). However, oddly enough, Kryptonite radiation only bodes harm for Kryptonian creatures (except for White K and Blue K, as described below). Luckily for Superman and other Kryptonians, just about any intervening amount of lead -- even as thin as tinfoil -- can completely block all Kryptonite radiation; in Action #278 Superman builds a lead suit that he continued to use on occasion through the years.
When passing into our atmosphere, Kryptonite "didn't burn up in Earth's air like ordinary meteors because Kryptonite can’t combine chemically with oxygen, which cause combustion" (Superman #130). However, as told in Action #283, Action #412, and other texts, Kryptonite can be chipped, melted, and otherwise manipulated. Thus, it is clear that Kryptonite is the one remnant of Krypton that does not become invulnerable in Earth's environment. "As we have shown in various stories throughout the years, Kryptonite can be broken, melted, pulverized into dust, and dissolved by acid" (Action #296 lettercol). Lois Lane #28's lettercol -- from the year 1961 -- informs us that "Kryptonite fragments lost their invulnerability when they passed through the green cosmic cloud which made them harmful to people born on the planet Krypton." Notice how that statement, danger-caused-by-cloud, contradicts the text from Superman #130, two years earlier. As time went by, the settled DC doctrine became that the basic, damaging effects of Kryptonite on Kryptonians was caused by the nuclear explosion making the stuff radioactive. As time went on, and other varieties of Kryptonite were introduced, the idea was promulgated that the difference in colors and effects were caused by different colored space clouds, which changed an already dangerously radioactive element into various forms with different effects on their hapless Kryptonian victims.
Now -- down to specifics, where we may find as many exceptions as there are rules!

Green Kryptonite. The grandpappy of 'em all. This is the variety first encountered by Superman when he was eight or nine years old, "in the very first year of my career," as told in a tale reprinted in Superboy #165. Found by Pa Kent near where Kal-El's rocket had crashed, it was put in Clark's room with his mineral collection. Soon the Kents were watching in horror as their supposedly invulnerable son weakened and sank into the delirium now known as Kryptonite Fever. Only the accidental interposition of Clark's first Superboy robot, whose inner workings contained lead, saved the Boy of Steel.
From time to time, criminals have used Green K's lack of harmful effects on humans to cruel advantage. (Yes, before the reboot in 1986 of Superman, Kryponite's radiation had no effect at all on anybody except Kryptonians! -- except Blue and White K, as listed below.) In Action #249, Lex Luthor swallows a serum derived from a dissolved Green K meteor to become "The Kryptonite Man." In Action #349, a Dr. Kane takes a Green K transfusion, enabling him to also exude deadly Kryptonite rays.
Many "older" fans may fondly recall the Kryptonite Kid, who with his pet dog passed through a Green K cloud in space that gave them Midas-like transmutation powers. In his first encounter with Superboy in Superboy #83, he first turns Smallville High into Green K, then Clark's entire block! Only the intervention of young Master Mxyptlk saves Superboy and Krypton. The Kid and his green mutt return in Superboy #99, but Superboy tricks them into flying through a Red K cloud that turns them temporarily good (!). Next, the Kid appears in an extended dream sequence in Superboy #128 and is not seen again until, as a grown-up Kryptonite Man, he expires in a mutual death struggle with Krypto in Action #583's "Whatever Happened to the Man of Tomorrow?"
In 1971, Superman #244-242 told a long tale that was intended by editor Julie Schwartz to strip away some of Superman's more godlike powers. One of the plot factors involves an experiment gone awry that somehow turns all Green K on Earth into harmless iron. However -- as later tales illustrated -- there was still plenty of Green K in outer space. For instance, World's Finest #196 tells how "a gigantic meteor shower covers the entire country" with enough Green K to fill a boxcar.
More Green K factoids: Many texts have established that Kryptonians cannot build up an immunity to Green K (Action #262), but on occasion Superman has hypnotized himself to feel and show no pain during brief exposures (Action #278 and 377).
The boy Lex Luthor created a successful Green K antidote that now is lost (Superman #173 and 292); and by 1959, Superman calculated that he had tried 855,019 different formulae for a Kryptonite antidote -- without success.
A 1961 H-bomb test sent Kryptonite into many time eras in the past (Action # 274).
"Just as there is 18-karat gold and 14-karat gold, Kryptonite comes in various 'proofs,' ranging from 100 percent to 30 percent" (Action #310 lettercol). Jimmy Olsen #81 tells how aliens imprison Superman in a force field, demanding Green K as ransom. But Jimmy outsmarts them by collecting "at least 50 tons of the deadly stuff! … I figured that just as a small amount of electricity can kill someone, but a colossal charge can't … a super-abundant amount of Kryptonite might not kill Superman!" And, jeepers, he's right!