How to detect when screen is white

Hi! :slight_smile:
Im trying to make a strong led light flash when my screen is totally white.
Do you know how i could make this work?
I have already seen the rpi ambilight, but that is messy and difficult to program.
I whould prefer using a arduino since it costs less.
Help?
Thanks :slight_smile:

(deleted)

Sorry,
I meant a monitor
Computer monitor
:slight_smile: