Sounds like a bad case of "dead tube." Basically, in the case of where a monitor changes to different colors, one of the three electron guns inside the monitor is on the blink or has died. Monitors work by basically combining the three primary colors - red, green, and blue hence acronym (RGB), to produce the multi-colored images on your screen. Like a TV, the small particles fired at the screen "stick" to the screen itself, allowing the illusion of a "fixed" picture. I think that's about it. Basically, when one of your guns dies the other two combine to produce weird, tripped-out colors...
Someone correct me if I'm completely and blatantly wrong here...I've found this to be the case on most older, crappier displays I've owned...