Do you have a loudspeaker next to your monitor? Some speakers, normally those not made to be placed next to monitors, will mess up a CRT screen with magnetic fields. This won't go away though, as your friends apparently claim. Check if your screen has a "get rid of magnetic interference button", you usually find this in the main menu of your monitor, often with an icon of a crossed over magnet.

If that won't work, move the speaker (or whatever it is that's interfering) away from the monitor, you should see the colors change on the screen immediately.