How to invert the colours in iOS 12 for iPhone/iPad/iPod Touch

• 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 feature can be used in combination with other accessibility settings such as ‘Colour Filters’  and ‘Increased Contrast’.

1. Open the ‘Accessibility Settings’: Settings > General > Accessibility. (See the article Open Accessibility Preferences on this website for more information.)
2. Under the ‘Vision’ section, tap on ‘Display Accommodations’.
   
3. Tap on ‘Invert Colours’.
   
4. Tap on the toggle switch for the ‘Invert Colours’ mode you would like to use:
   • Smart Invert
   • Classic Invert
     
5. The display colours will change immediately.
   
6. When you are happy with your settings, either tap ‘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.

More information:

How to enable 'Increase Contrast'

How to enable 'Colour Filters'