system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] So why DOES Kryptonite harm Superman? And why does it come in such a wide range of colors?

His Fortress of Solitude or his spacepod don't hurt him, even though they're presumably made from Kryptonian materials. Other meteorites don't seem to do anything, even extrasolar meteorites. What is it about the specific nature of Krypton or the circumstances of its destruction that caused it to, a) glow in a variety of colors and b) have such a range of effects on Kryptonians and, for the most part, only Kryptonians?
gpt
It should be noted Superman isnt inherently strong nor have powers.   He gets his strength and powers primarily from our yellow son and (in certain canon) Earth's atmosphere.   If he were on the planet Kryton, with their red sun, he'd be just as average as anyone here on Earth.  (no one on Krypton has super strength, nor can fly, nor have heat vision, etc)

Green kryptonite pieces are radioactive portions of Krypton which radiate that "natural" Kryptonian effect onto him and undoes the benefits he gets from our sun and atmosphere.  The destruction of Krypton, IIRC, is what makes it radioactive.  Other pieces of kryptonite we see are pre-destruction, or pieces that did not become radioactive (and thus isn't green).  In smaller doses as we most often see, t's not so much that it hurts him, as it brings him down to a level he's not accustomed to.  Granted, in large quantities and because it IS radioactive, it could potentially kill him.  Keep in mind, he's never really lived on Krypton.  He was shipped off right after birth.  The only environment he knows is Earth's.  His body grew up acclimated to it.  That said, Kryptonite has much stronger affect on him that it would say, Zod, or someone who lived for many years on Krypton.