What model iMac...?
see > Find the model name and serial number of your Mac - Apple Support
oakharoh wrote:

This is what my display looks like... Currently set on 16.5' (1920 x 1200).
Unfortunately, that looks more like a graphic's problem than a color or accessiblity setting.
2 days ago, my computer was very hot to the touch. Shortly afterwards, it beeped a couple of times and then went off.
That is a pretty good indication that it over heated.
By the way, the computer has had loading issues where it spends close to 10 mins on the apple white loading screen.
That sound more like a hard drive problem, which most defiantly could also be cause by or attributing to hi temperatures.
I'm not sure, but it seems as if either the air-intakes are clogged with dust or that a fan has failed. Which unfortunately, may have already caused irreversible damage to both your Hard Drive and Graphic's chip.
So at this point, sadly I can only suggest that you take it to service.
Mac desktops Service Answer Center - Apple Support