First go to settings.
Then scroll to general.
Next go to accessibility.
Then simply turn invert colors switch on.
To bad it doesn't stay invert colors when you screenshot!:(
You can also set it to when you triple-click the home button it turns to invert colors by clicking the triple-click button which is also under accessibility.
Then click invert colors to make it gave a checkmark by it. Then wellah you have triple click invert colors!
The creator of this guide has not included tools