Nail polish if it's in your house and at hand and you won't ruin the brush (or use a little hobby brush).

I use hobby store paint because the bikes I've touched up are blue and my kids don't have blue nail polish! Hobby stores have so many options, the little bottles (i.e. Testor's) are cheap, they have cheap brushes right there. Look for a close match, but don't be afraid to mix a few drops from a couple of bottles on a piece of paper to blend to suit.

By the way, Michaels - a common chain - has hobby paint and brushes. Look for a lacquer or enamel.

Me? for small chips, I've just gotten close to matching the color, and from further than 3' away, they're invisible.