Quoted:
I still have a decent supply of "Testors" model paint pens from way back when I did the model plane/car thing. I do not know if you can still get them or not. They don't carry models around here anywhere anymore. Best bet would be a hobby shop of some sort. Touch up pens like the ones they carry at Auto parts places should work too.
I wouldn't recommend these. I just bought one to touch up a bunch of tiny rock chips on my truck and the paint can really flow after you do one dot. It seems to gather up on the ball point and form a decent drop just waiting to drip <––- I hope that makes sense.
I used to build model rockets and also used Testors products, I think those pens would work much better as the point is finer and flow is a little more controllable.