I have tried the "invert colors" option and it made many things even more difficult to see.
iOS7 is absolutely dreadful in terms of visual accessibility! I have sent feedback to Apple using the form here:
http://www.apple.com/feedback/iphone.html
The only visual impairment I have is that I need to use reading glasses (about +2 strength) but I have had to turn on all the visual accessibility features to be able to be able to use iOS7 at all.
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Apple used to be the leader when it came to accessible design - they have thrown all that away and vastly reduced their target market with this unbelievably poorly designed UI.
If they do not release an update with options to improve visual accessibility then many existing iPhone owners will have no option but to look for jailbreak solutions and void the warranty on their phones simply to be able to use them!
I have used an iPhone since 2009 and bought a new iPhone last week. If I had known that iOS7 was going to be like this I would have looked around to see what the competition had to offer.