sadly, once in a while a bike will get scratched. it would be nice to cover the scratch with a little bit of touch up paint. in this case i could use some glossy black. for a metal part.

any advice on what to buy?

Gloss black shouldn't be too hard. I'd get a little bottle of model paint from the hobby store or try to find matching fingernail polish (without the sparkles) from Walgreens or someplace like that.

Be aware that there's "perfect match" and there's "not exactly". "Not exactly" looks like crap because it draws your eye to the blemish. Clear beats "not exactly".

Testors enamel paint or good old fashion nail polish. I have used both on my bikes.

My LBS has a box full of little bottles of brush in the cap, touch up paint, that were included with the boxed bikes from The exporter,

But typically are too much to hang on the handlebars with the price tag for floor display and test rides..

My new bike came with a bottle of touch-up paint. At first I didn't know what it was - grease, oil, thread locker? Paint! Never heard of such a thing.

There is also touch up paint and clear coat in small bottles in auto parts stores. Gloss will probably require some clear coat to obtain.

Enamel paint from a hobby or craft store like Michael's works great if you can't get an exact match from your LBS. Small bottles are less than three bucks and offer plenty of paint for touching up scratches. They will also have some cheap brushes in a multi pack for a couple bucks as well. You should get rid of any loose paint along the scratch and then take a tiny bit of sandpaper to remove any oxidation on the metal. If it's steel and there's rust, definitely sand it away. Aluminum oxidation isn't as much of a problem but I would still sand it. Just wrap a bit of sandpaper around a nail and you can minimize the area you are sanding. Wipe away any loose dust from sanding and apply your paint. It's best to do a number of thin coats than one thick and heavy coat. You can add clear coat if you want but often it's not necessary with a decent gloss enamel.
Good luck.

Unless you can get the EXACT paint the manufacturer used, I suggest not bothering with colored touch-up paint and going the clear nail polish route. (I once bought a frameset and the manufacturer supplied a small bottle of touch-up paint along with it, which was thoughtful.)

In my experience, even when the color is pretty darn close at time of application, differences can appear later as the touch-up paint reacts differently to sunlight, sweat, pollution, etc. (I am speaking from experience.) You might have better luck if your frame was painted to begin with (and you can lay your hands on it), but if the frame was powdercoated originally and you're like attempting to patch it up with paint, you'll likely be disappointed.

And black...well, let's just say that Testors' Gloss Black proved to be much grayer than my fork's gloss black paint (that time, the fork was actually painted), so now I have ugly gloss grey-black patches on my gloss black fork.

Mike