A CRT monitor is vulnerable to stray electrical and magnetic fields. Though most monitor manufacturers shield their monitors from these fields, the shielding is not very strong. Prime culprits for these kinds of distortions are speakers.

They contain powerful magnets and though they are also shielded, placing them too close to your monitor can cause the picture to get distorted and discolored. Power supplies of computers are also possible sources of electromagnetic radiation.

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The color of the picture can be permanently distorted. To reset your monitor, you need to use the Degauss option. Refer to your monitor manual to find out how to select the "Degauss" option. While degaussing, the monitor flashes and blinks for a while before the image stabilizes. Remember that if a slight discoloration still persists, don't immediately degauss your monitor again; you risk damaging it. Wait for at least half an hour between degausses.

If degaussing a monitor doesn't work, it means that that portion of the picture tube may be damaged and the tube may need to be replaced. Since the picture tube is the most expensive part of the monitor, it is advisable to buy a new one.