Every time you switch on your TV from cold - that's not stand by - but with the big switch on the front which puts the light on, your TV will de-magnetise itself to remove colour stains on the screen, as the set manufactures know that magnetic fields can appear from a number of source - including the earth.
If the stains are bad, it might take two or three switchings to clear them - give the set 15 minutes or so to cool down before you do another one.
If they don't clear then you need a deguassing coil which you rotate in front of the screen to take away the magnetic field.
You would have to had very powerful magnets around to do any lasting damage!