I had to fix (twice) the iPad of one of my fellow travelers on my trip to Morocco recently--she had somehow managed (TWICE!) to click the setting for "invert colors." So her whole iPad looked like a color negative.
Thankfully, I found the setting under "accessibility"--which I'm sure is the basis for the "high contrast" setting, as well. For people with certain vision impairments.
So yeah, I feel your pain. It took me a while to find the setting for her problem.