Green Items are "Set-Items" , collect them all ! <img src="/ubbthreads/images/graemlins/delight.gif" alt="" />
Seriously , they create some "synergies", benefitting the char more, when all of them (or at least two) are available. They are still very rare.
Very rare are also items wth golden (not yellow) names : These are among the most rarest ones, the so-called "Uniques".
Using a Topaz in socketed items can incease the possibility to find magical green ones and golden ones (concerning theletters of the names) , too. Read carefully what the gems descriptions say.
-> Area Summit you can read everything ! Check it it if you have any questions !