i have not had good experience with paint pens. i've never been able to make the nibs last long, and they are expensive to replace. 
 
a parts dept. dealer actually recommended me to go and have a small bottle mixed up at concept finishes in richmond. they are on bridgeport rd. west of garage five. 
hopefully you only have a single-stage paint finish because if it's 2-stage you have to apply clear separately and that gets complicated for DIY. 
when i asked this question on RS, i was given this link:  
http://www.autopia.org/publish/artic...hes/Page1.html 
i've also used 
http://www.langka.com to clean up my mess afterwards, but it's been a hit/miss for me in getting good results