Without sounding *too* unhelpful you would probably want to calculate who should be flashed by it, rather than trying to get who actually was.
No, wait - sod that, this must be quite possible; one of the servers I play on has a sound effect for when you get flashed so there must be an event there somewhere.
EDIT: See V3x's "Colored Flashbangs" mod -
http://forums.alliedmods.net/showthread.php?t=15241
The source is very small, and you should be able to work something out from that.