Contrast/brightness issue for only one of the iMac's users

There are two users on our iMac and one of them has suddenly started experiencing a bizarre problem where the colours are too white

It's almost like someone has removed the colour and pushed the brightness up (though the screen isn't any brighter and I've checked the brightness levels for both users)

I've tried logging off/rebooting but this hasn't solved it.

What other possible causes could there be?

An example of how it manifests is that when I select the other user/login in box, I see the colour of it as the standard apple 'grey' but when they do it it shows up as white