If the threads of the bb shell were damaged when the bb first became loose, loctite will help keep it in place. You want to use the lightest available. I think the purple is the lightest. The blue will work as well, but will make it extremely difficult to remove since the bb threads have so much contact area (large diameter, fine pitch threads). The idea of locking compounds being a bad idea is pretty much nonsense. When you are installing the fastener, the locking compound will act like a lubricant, letting you torque the fastener to the right specs. Once it cures, it will help keep the fastener from loosening. A properly torqued fastener shouldn't loosen, but damaged threads, poor tolerancing, vibration and installation mistakes can all effect a fasteners ability to hold. Loctite is a nice safety measure.