I use a siphon-feed Paasche H for spraying primers and top coats. After a paint session, I hold the AB vertically for a moment so that the paint in the needle can flow back into the bottle. However, as I take out the bottle, a few drops of paint almost always comes out of the needle and create a mess. Is there a way to prevent this???

What you said is true with any single action siphon fed airbrush . It's just the nature of the beast. I have a badger 200 NH and it does what you described, I just try to mix just enough paint so it's all used if not got to have a paper towel ready just in case .now a double action airbrush the needle closes all the way so no problem. Only way to solve this issue is to change the airbrush