Which parts needs to be replaced when phone screen shows diffrent colo

My son recently dropped my phone (Galaxy SIII GT-i9300) in water and was working fine after it dried up but after that it would show on the screen different colors and would not boot. Is there a part that is damaged or needs to be replaced cuz of water damage? And if you know what is damage where I can purchase that? I was looking for the motherboard thinking that is the main problem but on your site you dont seem to sell that.