I've always wondered the same. Thing is I only have the top button. When pressed it remains red and doesn't go yellow. Not quite sure how to tell if it's on/off!
When I had my car in the workshop yesterday I only noticed these because I was vacuuming the car out and they where illuminated red so I wondered what they where
so I pressed them as you do
and they turned yellow, but when I pressed them again they just stayed yellow
So when I turned my ignition on and off again they went back to red, so I assume red is it all ok and yellow is the alarm turned off