I use the pens from halfrauds. They are much easier than the brushes and the nib follows the scratch exactly.
I agree with the comment above, less is more, but you need to lacquer it on top if you want the paint to last, otherwise it will just wash off eventually.
Halfrauds do a set for the fabia black magic paint which is a close enough match for scratches and chips. Afterall it will never look great, but at least its better than nothing.
My main reason to touch up the paint is to stop rust, not just for asthetic reasons.