When I triple-click my iPhone home button, the colors on the screen invert. Check it out:
This dazzling trick is one of several functions that can be assigned to the iOS accessibility shortcut (see: Settings / General / Accessibility / Accessibility Shortcut). Along with Invert Colors, other options include the ability to turn on or off VoiceOver, Grayscale, Zoom, Switch Control, and AssistiveTouch.
While these functions are designed for a narrow user base, some may appeal more widely, and they're all more useful when they can be turned on or off in an instant (try assigning only one function for maximum speed).
I'd recommend Invert Colors. Aside from looking cool, this mode is useful for reading in a dark room, as it makes the phone give off less light — which not only helps other people in the room sleep but also helps you sleep, given that light from devices has been shown to screw up your sleep cycle.
Zoom, which lets you magnify parts of the screen, may appeal to many people. As for the others: VoiceOver reads text on the screen; Grayscale removes all color; Switch Control cycles a cursor around available input fields.
You might as well pick a special function — and at least one is actually pretty useful.