Your tv has a rainbow effect?

This coloured effect is caused by residual magnetic fields on the shadowmask of a cathode ray tube tv. When a tv is switched on an alternating electrical current is passed through a coil looped around the crt face, this eliminates these stray magnetic fluxes. There may be a fault with this circuit. A magnet brought into proximity with the tv screen can permanantly magnetise the shadowmask or flex it out of position.