a paint pen is the best thing to color on your custom clones because it draws smoothly and colors over things.
the paint pen will not work when you open it. you have to shake it a good bit. the cap should be on while you're shaking it - to keep the paint from shooting out! to get it to start working, you have to press the tip on a piece of paper a few times. a little bit of paint may shoot out of the tip. and then it starts working.
when you use a paint pen, your clone will look like it was made this way (as if it came from the lego factory).
be careful, you might make mistakes. paint pens are permanent! fingernail polish remover will NOT remove the paint (you might have to use crazy chemicals from your gramps to get it off!!!)