[HarryT]

A diffraction grating is basically an optical device for splitting up and spreading out light. In this case it's used to spread the light from the 4 LEDs at the bottom of the screen evenly across the whole screen. But as well as spreading light out, a diffraction grating also splits light up into its component parts - the "colours of the rainbow". I'm sure the PWs light diffuser will have been designed to minimise this effect, but it's bound to happen to some extent or other even so.