Although rare, it was the most common type. It was harmful to natives of the planet Krypton. If one was exposed long enough, they could go into a Kryptonian trance or it will turn their skin green from Kryptonite poisoning, and not long after it could kill them. Typically, by the time the skin was green, it was too late. Villains constantly used Green K as a weapon to defeat Superman. Green K was a key element in DoctorNataz'Noxiumcrystal.
Although Red K had no fatal effects on Kryptonians, it's effects were still unpredictable. It could give Superman extra arms, or it could even turn him into a child. It could do a ton of other things. Red K is unpredictable, there's no telling what may have happened. But one thing about these unpredictable effects, was that they were only temporary, usually only lasting about a day.
Blue K wasn't harmful to Kryptonians at all, in fact, when injured, it could heal them. However, it's effects on Bizarros (such as Bizarro himself) was deadly. It could be created by using the faulty duplicator that produced Bizarro on Green Kryptonite, although naturally-existing pieces appeared in a few instances.
Purple Kryptonite was a type of Kryptonite that was purple in color. Like brown Kryptonite, it is unknown if had any effects on Kryptonians or anyone else for that matter.
Krypton Steel was another form of Kryptonite, that was also presumably a very rare form. It was basically Krypton's version of the Earthmetal known as "steel." It was harmless to Kryptonians, but it was also indestructible to anything except for Superman, or possibly another equally powerful being. Some forms of Krypton steel however, are even indestructible to Superman.
↑(Although this seems unlikely, since Solomon Grundy and Bizarro are both roughly the same strength as Superman, and yet they didn't even attempt to open a box made of Krypton steel. Instead the Legion tricked Superman into doing it for them)