Once you get the rust removed by one of the methods given, fingernail enamel in the color that matches works just fine, or use Testors enamel or Model Masters model paint in the small bottle. The problem with some of the paints mentioned is finding a color that will match your bike paint color, so unless you're willing to repaint the entire bike I would suggest a much smaller project and use the Testors and a good quality fine paint brush using several thin coats. Testors is a very high quality paint too, it's durable and dries to a smooth finish, some people will put 3 or 4 drops of mineral spirits to the paint and mix, I never found that necessary painting very small areas and with a new bottle, older bottles that sat around while yes. But you can tell if you need to add the mineral spirits if the paint doesn't level (I would add one drop of thinner test on a piece of metal, if the brush marks level fine if not add another drop and retest, and do this till the paint levels, level means the brush marks smooth out and disappear by themselves; like I said painting very small areas found on bike related touchups I haven't had the need to thin the paint.
If you can get your hands on a airbrush paint gun that would work superbly, but with Testors you have to use 3 parts paint and 2 parts thinner for a gloss effect or 1 part thinner for flat (not sure about the ratios with Model Masters since I've never use that paint), use 23 psi again using thin coats till the color is solid. 48 hours is the typical curing time on metal.
Some will say to wet sand between coats, while that is the best way, I've never really found it necessary on small touch up stuff, cars yes, bikes no.
I've found that Testors brand of Premium flat paint brushes works just fine.
