Take the bike/frame to a local auto body & paint shop.

For approx. $25. they can shoot the paint with their electronic paint match scanner.

They can mix up an ounce or two of matching touch-up paint in liquid or rattle/spray cans.

I'd go for the matching blue (and/or white) in liquid; get a small paint brush from a craft store.

It is really a simple process, the results are excellent for cheap money.