Brown is ground, green is the brake light and gray is the tail light. It doesn't affect the spark on this bike though. Basically all you need for spark is to connect the black stator wire to the positive side of the ignition coil, ground the body of the coil to the engine or chassis. Ground the body of the spark plug some bare metal on the engine or chassis. If you don't have spark this way your issue is likely in the points or condenser. Coils can go bad, but they're not usually the problem. I'll usually slip some fine emery paper or a points file through the window in the flywheel and clean the surface of the points, then follow up with a bit of rubbing alcohol on a piece of card paper to clean them up afterwards. Once the paper comes out clean you're good to go. If you do have to pull the flywheel to replace the points or condenser use the correct threaded puller. It'll cost you just under twenty bucks, but it's cheaper than replacing a flywheel that got mangled by a jaw type puller.