In iOS 11, Apple changed this feature to offer two different options:
Smart Invert: Reverses all screen colours except images, video and some apps that have a dark interface.
Classic Invert: Reverses all screen colours including images and video.
This section takes you step-by-step through inverting the screen colours on your iPhone, iPad or iPod Touch running iOS 11, with pictures of the screens to help.
1. Open the Accessibility Settings by tapping the Settings icon, then General, then Accessibility. Look under the Vision section and tap Display Accomodations.
2. Tap Invert Colours to see the Invert Colours options.
3. Tap on the toggle switch next to the mode of Invert Colours you would like to use. You can pick either Smart Invert or Classic Invert. The colours will change immediately.
4. When you are happy with your settings, either select Back in the top left of the screen to return to the main Accessibility settings screen or press your device’s Home button to return to the home screen. On newer devices without a Home button, swipe upwards from the bottom of the screen.