Use the Accessibility Shortcut. Press the Home button quickly three times to turn any of these features on or off:

• VoiceOver
• Invert Colors
• Zoom
• Guided Access (The shortcut starts Guided Access if it's already turned on. See Guided Access.)
• Switch Control
• AssistiveTouch

Not so fast. To slow down the triple-click speed, go to Settings > General > Accessibility > Home-click Speed.