This is a rear-projection HD-TV using CRT technology, which means there is a conventional CRT projector inside with a large mirror to reflect the picture. CRTs show color by using a combination of three electron "guns", one for red, one for blue and one for green. If your screen has gone purple-ish then for some reason the green gun is failing.

It could be the display circuitry feeding the green signal to your CRT, but most likely it is the CRT profector itself that is now unable to use the green gun.

If the picture is occasionally good, maybe when you first turn on the TV (try unplugging it from the wall socket for a couple of hours first), then it is more likely to be the input circuitry.

If the picture is bad all the time regardless of what you do then it is probably the projector.

Either way it means a trip from a TV repairman. Hopefully it's still under warranty.

Hope this helps.