I'm not entirely sure, but I do know from experience that red light will blow out your goggles (like if Joe infantryman in the back wants to try and read his map in flight). I assume it's something to do with wavelengths but I'm no scientist.

Most military night vision goggles have a green filter on them so you can use a green light and not have your goggles shut down. Particularly helpful in the cockpit if you're flying with goggles on and need to look at a map or something inside