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.