Inverting the colours of the interface
“Invert colours” option
This accessibility feature allows you to toggle the display in negative colours for any application.
The “invert colours” feature can be activated through the accessibility menu from the Settings application.
Inverting colours work on all screens: Home screen, lock screen, Spotlight, and applications bought from the App Store.
Since iOS 6, this feature works perfectly with VoiceOver.
To improve readability, you can modify the intensity of the contrast through the Brightness setting.
Brightness Settings and Wallpaper
For iOS 8, the feature to invert colours will have additional display accommodations such as grayscale or inverted grayscale.
This will allow you to display the screen without colour, and to help certain people with visual impairment feel more comfortable.