A finger (or stylus)
Step 1: Settings
Tap the settings icon and go to General. I give credit to doodle buddy for the arrows.
Step 2: Getting There
Now scroll down and tap Accessibility.
Step 3: Yay
Now tap the move the slider that says Invert Colors.
Step 4: Or...
Instead of permanatly inverting colors, you can set triple click home to invert colors so every time you triple click the home button, you can toggle invert colors. Enjoy!