So a friend gave me a (2007?) iMac 24″, the whole screen had a yellow tint to it. I disassembled the LCD screen… And found the cause.
There is an acrylic sheet in the backlight that acts as a diffuser for the CCFLs. This acrylic sheet had aged, and the heat from the bulbs caused the whole sheet to become a bit yellow. The sheet became brittle enough to break on me while handling it for a bit. (Whoops!) Well I need to find a replacement anyway if I want the screen to look normal again.