Have a look at the colour chart on this page click here.
It is my suspicion that you will find one of the saturated colours (the ones with one colour at 255, and the other two at 0) is missing. If I'm right, then it is my guess that you have lost the connection to one colour in the monitor cable. Another possibility is that two colours have shorted together which will result in one saturated colour being correct and two others wrong. I have this problem if my monitor cable is sharply kinked.