Faulty smartphone

A simple application for smartphones (would probably need to be rooted/jailbroken for access to low-level hardware controls) that would simply turn the whole display to greyscale. Basically as if you took a screenshot of your phone and desaturated it in GIMP/Photoshop/etc. Your whole phone would be entirely functional except for lack of colour on the screen.

If you needed full colour for a colour-dependent action (camera, looking at pictures, etc.) then you would simply slap the phone a couple times, as one used to be able to smack old tube TVs to get them to work (Fonzie and the jukebox is a good example). Once the application picked up the appropriate accelerometer data, it could "flicker" the screen back into full colour, as if smacking your phone had actually solved the screen "problem".

Of course the application could be turned permanently on/off at will, as well as have a few modifiable settings such as accelerometer sensitivity (shake to fix, slap to fix), colour settings, etc.

Would be a cool application to get some confused looks from friends, as well as the timeless novelty value of hitting something to make it work.