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.
If you boot the Mac holding the option key, this will have the firmware drive the screen and isolate any software corruption or odd driver bugs in most cases and let you know it's a hardware issue.
Usually vertical lines are the LCD failing - then a cable issue - lastly the GPU.
Usually horizontal lines are a cabling issue - then a GPU issue - lastly a LCD issue.
That's not to say you even have a hardware failure. Do post a picture and we can refine things based on the actual failure.