You can take fingernail polish (matching) and dab it lightly. Or, you could stop at a body shop and they probably have a little extra paint in a matching color left over. You can get a small bottle of it from them (they probably have 'touch up' bottles on hand). It won't look perfect of course, but it will be much less noticeable to anyone else, and will stop it from rusting.
Getting it repaired the proper way, will be expensive. And unless you know what you are doing (proper color match, and blending technique), doing it yourself would be tricky and still expensive. Paint supplies are pretty darn expensive.
Oops, I totally missed you saying you had some paint. Ok this is gross, but it really is the best way. Find a cigarette (or butt) and just cut the very end of it off. Dab that in your paint and on the vehicle, with very little at a time. Too much is bad. A cigarette butt leaves better looking results than a brush.