Also, check your video card.
I had a Gateway 486 system a few years back and the Gateway CrystalScan monitor it came with developed a similar problem (mine was yellowing, not greening) toward the end. I still have the system (gathering dust in the basement) and I gave the monitor to my son, (he's little enough still that ANY system, ANY monitor will wow him), but on a newer system with a decent (though integrated) ATI video chip and the discoloration problem disappeared for the most part!
But I'd also pay attention to what Terradyne said. There may be more problems with your old monitor than can be corrected by tweaks or video cards.
The big-name online computer catalogue houses often have some big-name 17"-19" monitors on sale at decent prices, especially the 17"s. I hope you find a good, cheap replacement.