I love the cobalt blue of the Bawls guarana bottles, and I wanted to add a shimmer to the inside of the glass to enhance the blue. So I sprayed the inside of the bottle tops with Krylon mirror-like paint. In order not to get paint on the outside of the glass, you have to trace a circle on a piece of cardboard with the bottle top, cut out the circle, and place it over the opening of the bottle to protect the outside from stray paint. Spray a fine coat and let it dry. Repeat until the glass is the desired mirror finish. I sprayed mine with the coats of Krylon mirror-like paint.