This is more likely to occur at a corner or edge of the screen. The magnetic fields cause the electron beams that comprise the picture to be distorted, causing colored areas to appear on the screen. The magnetization normally occurs from interference created by an electromagnetic device, but can also occur if the monitor is shocked or moved suddenly. The screen must be degaussed to get rid of the pattern. Degaussing is the process of demagnetizing the surface of the CRT screen .
Many monitors automatically degauss the screen each time they are turned on.
You can tell if your monitor does this by listening when it is turned on. If the monitor is warm, you can see the screen image warble as the degaussing is performed. If your monitor automatically degausses, then after removing the source of magnetic contamination, you may not need to do anything to remove the colored spot. After turning the monitor on and off many times through normal use, the spot should disappear. This should take a week or two.