I find it very convenient to touch up with Testors or Tamiya enamel markers. But those markers don't have a fine tip. Does a fine tip enamel marker exist???

Try sharpie they make a fine point oil based opaque paint marker , not sure what colors AC Moore carries them .

look for paint markers in the art supply store, there are as fine markers as 0,5mm with pigmented paint ink. I use a japanese brand (Uchida) which is available here but there are a lot of brands. However the color selection is rather limited compared to hobby paint lines.

Mr Hobby makes the Gundam paint pens also .

Sharpie's or other ink pens like permanent markers are good, but bitter experience tells me you need to let them sit for 24 hours before you varnish over them (oops).

Regular Sharpies will bleed through a top coat. I've bought a few Sharpie oil-based paint pens and they won't bleed. They seem better than enamel pens.