Recently the picture my Sony flat screen TV has started looking quite green - does not matter if we are viewing TV or DVD - everything has a greenish hue now. Its about 5 years old and out of warrenty. Is this fixable for a reasonable cost?

The set is probably magnetized and needs degaussing (demagnetizing).
All CRT type TV's have an internal demagnetizing circuit that is engaged when the set is first turned on after being off for the day.
If this circuit is working then you sometimes hear a "bonk" sound (about 1/2 a second long) when the set is first turned on. If this circuit isn't working the set will become magnetized over time and will have to be degaussed using a degauss coil available at a TV repair shop (cost aprox. $35.00) and you should have the circuit checked at the same time. A set will also become magnetized if a large speaker (magnet) or motor is close to the TV. Be sure to vote/rate me if this helped Thanks!