There's "perfect match" and there's "not exactly". "Not exactly" usually looks like crap because it draws your attention to the paint chip.

Fingernail polish comes in a LOT of colors so, if you have a red or a black bike, you have a decent shot for a good match. Testors has many fewer colors but, if you are a good color mixer, that might work for you.

I generally just use clear fingernail polish. I don't know how much metal protection it provides, but it makes me feel better.

If you have a lot of nicks or a big area to do, my advice is to have the whole frame powder coated. I've not had good results using a rattle can over existing paint.