Hola Pablo,
Painting lug lines (striping as well) so they look good is a skill only a few possess. I suppose if you found an artisan that paints ceramics or other items for tourist trade or paint artist you might have luck and have a satisfactory result. I would give the artist a junk frame first to see how it comes out plus for practice before attempting on your good frame.
I did this once many years ago using enamel (Testor's model paint or similar) and a fine natural bristle brush but the results looked sloppy and amaturish.
The best (finest) lug-lining I've seen is not the type that simply follows the outline of the lug...but is separated by a few millimeters giving a more dimensional quality as well as emphasizing the lug outline. I can send you some pics of what I mean if you wish. Of course, separated lug-lining like this won't cover-up on-the-outline lining that already exists so your only options may be to leave as-is or paint over.
Jack Romans
Sacramento, Califa