How to Turn On White on Black on Your iPhone
If you are an iPhone user who has visual challenges, the black-and-white screen option offers a black background with white lettering that some people find easier to use when reading text. The White on Black accessibility setting reverses colors on your screen so that backgrounds are black and text is white.
To turn on this feature, tap the Settings icon on the Home screen.
Tap General and then scroll down and tap Accessibility.
In the Accessibility dialog, tap the White on Black On/Off button to turn on this feature.
The colors on the screen reverse. Tap the Home button to leave Settings.
The White on Black feature works well in some places and not so well in others. For example, in the Photos application, pictures appear almost as photo negatives. Your Home screen image will likewise look a bit strange. And don’t even think of playing a video with this feature turned on! However, if you need help reading text, White on Black can be useful in several applications.
For more help on iPhone accessibility features, check out iPhone Accessibility Options for Easier iPhone Operation