Ride your bike to a hobby store and try to match the bike paint with Testors enamel. You might need to purchase 2 bottles to mix for matching the paint colour. Half an ounce is probably all the volume of paint you need so a bottle of Testors works great, even if it is $4 per bottle.
In Canada, we have a paint called "Trem Clad" that comes in spray cans and 8 oz cans of brush on. Lots of colours to choose from. In the US, a similar paint is called "Rustoleum" or something like that. You can sand the scratch smooth before you paint, but you don't need to sand down to bare metal unless you have Obsessive Compulsive Disorder, in which case you will probably want to do 4 coats after primer, and sanding between coats with progressively finer emory paper, finishing with a clear coat.
There, I got a whole paragraph of information into one sentence and I'm only on my 4th espresso of the day.