IIRC, white bands on the tubes were live, and yellow inert.
As far as gaming a PIAT ambush, I'd use some pre-programmed actions/reactions for the AI side. Of course, that will depend on if you're playing the Germans and the AI as British/Canadian, or vice versa.
If you are playing the Germans, then have a table you can roll a D4 or D6 on each turn a piece of the Germans' armor approaches a possible ambush point. The PIAT may not be there, may hold, may fire, etc. That'll make you as the German armor commander very wary, without knowing when/if a PIAT may rear it's ugly head.
If you are playing the PIAT, do something similar for each time the Germans approach a possible ambush point. They may proceed as planned, have infantry check it out, blast the spot with HE or MGs, stop and ponder things, etc. That way, you us the PIAT guy will have to be careful about siting your ambush.
Both tables could range from blissful ignorance to total tactical awareness. Maybe have a couple modifiers to reflect AI troop quality, your troop quality, type of cover, etc.
Just my initial thoughts....

I will say that in our games, Bazookas, Panzerschrecks, etc. come and go, but for reasons known only to the dice gods, our PIATs seem to always do yeoman's work....
