I'm guessing that the double yellow in the Chalfont & Latimer photograph is only cleared due to the white light being out. I would expect the signal to be at danger if there were a problem, because a driver is never going to see a little pidldy white light when approaching a signal - we're trained to 'target fix' on the aspect - and 'watch it through' until you're past it, in case the sigggy 'puts back' in an emergency.
Could be wrong though.
Cheers, John