Nail polish, model paint from a hobby shop, maybe online sources if you know the original paint color -- take your pick.
And for most scratches, a teeny-tiny skinny artist's brush with about three camel hairs. A 90% match on a scratch that's just barely filled with paint looks better than a 99% match that's smeared across 3/16" -- the latter looks like an amateur tried to repair it. And since most decent bikes have clear-coat over the paint, buffing it out gives you a cloud around that amateur smear that shouts, "Look at this!"