Either a lens is dirty or the light engine is bad. You can clean the lens yourself, or pay a few hundred to a technician. If the light engine is bad, it has to be replaced, and that costs as much as a new tv in many cases. Are you under warranty at all?
If it is just one color that is bad, I'd try to clean it. If there are 2 colors going bad, then its most likely the entire light engine/lcd, unless you can recall some freak accident occuring that would have filled the thing with dust.
Even if you do clean it, it's not easy, and may end up worse.
I'd start shopping around for a new TV. Repairing modern electronics isn't usually worth it, and if your past the warranty you probably got your money's worth already.