Yellow with Green stripe should be your hot wire. If you are using a ohmmeter or continuity tester to determine it is ground it is probably reading through the coils or something.
Yellow with green stripe is the hot lead for both mine and my kids bike. BUT, solid yellow is used for ground at some points.
Here is a generic schematic that pretty much holds true for our bikes..
http://www.gasgasrider.org/gallery//...13&limit=views
Do a search for "schematic" or "diagram", there are a couple members on this board that have supplied some really good wiring diagrams and they're here somewhere.
BTW, mine and my kids don't have headlamp switches on the handlebar. We both have the abbreviated wire harnesses with a toggle switch behind (attached to) the front number plate/headlamp.
One last thing, these are AC systems so make sure your DVM isn't set to DC. You will need the correct AC bulb also.