Do you have any speakers near the tv? If so move them further away. The discoloration is due to magnetism. Often occurs if you move the tv's location or angle. Also, may occur if the tv is near central heating radiators, power tools or magnets have been in close proximity to the tv screen. The set automatically demagnitises itself at switch on. In some cases it may require manual 'degaussing' by an engineer. If the sets auto-degaussing circuit fails it's usually inexpensive to repair. Permanent purity errors (very rare) can be caused by misalignement of the tube scan coils or physical shock.
Cobweb, September 2004