iMac display has discolored tan lines
This looks like a backlight uniformity issue.

This is where the screen isn't being evenly lit by the CCFL bulbs behind it. Some of them are outputting differing amounts of light and cause this perceived 'discolouration'. The lighter colours really do show it.

Unfortunately, this is one of the disadvantages of a CCFL backlight system as opposed to something like LED backlighting - backlight uniformity can become an issue, especially with time.

You'd be looking at a screen replacement - outside of warranty, that's gonna cost you.