Always a good idea to match the paint color and just get a small bottle of touch up paint for chips & scratches in the finish from the local auto store. Dupli-Color runs $ 7-8 and you can touch up as necessary. For my Vilano (gloss black), one of the welds had a sharper rough spot where the top and seat tube is. No big deal, I filed the sharp edge smooth and dabbed it with the universal gloss black, didn't even have to use any primer.

With surface rust, without a pic of it and as small an area as what I perceive you're talking about on the actual steel tube itself, you could probably just dress the rust spot with a finer grit sandpaper, maybe even a dish scrubbing cloth and dab it with a little gray primer on the end of a q-tip and then dab it with the pen tip or brush of the matching paint touch up container too.

Just me, but any blemish to the paint finish that goes to the metal gets touched up immediately after a ride, with humidity & moisture as it is in FL, rust starts within hours and becomes more readily visible after a day or two. Like any problem, take care of it asap and the cancer doesn't spread.