Flip-top dropper caps (Green Stuff World)Quick Flip Dropper Caps for Hobby Paints Green Stuff Worlds flip top dropper caps are a universal upgrade for most dropper bottles on the market with standard 15 mm diameter necks. The magic is the built in lid: a flip top hinge you pop open and shut with one thumbno threads, no loose parts. While your other hand keeps a brush or airbrush steady, you can squeeze the bottle, grab a drop of your favorite acrylic paint, ink, or enamel, and snap it closed