Empty the glue into a sink and wash the inside of the pen until it is dry. Let it completely dry before moving on to the next step.
Put on latex gloves before coming into contact with the paint. Wear old clothes you don't mind getting paint on just in case some of the paint gets on you.
Spray or pour some of the paint you want to use into the bowl. You can use any type of paint you wish, including spray paint. Add three or four capfuls of turpentine into the paint mix a couple of times while emptying the paint into the bowl to keep it from becoming too thick.
Use a paper or plastic funnel and pour the paint into the empty and dry glue pen. Add another capful of turpentine once the paint is in the pen.
Cover the opening of the pen with your fingertip and shake the pen a few times to mix everything together.
Screw both of the glue pen's caps back tightly onto the pen and you're finished.