Specialized sells bottles of touch up paints that match their frames. I bought a bottle for my 2002 Enduro this summer and it works well enough. Perhaps you can try to check your bike's manufacturer website and see if they sell a matching color?
Application techniques, go slow and small. Its really hard to touch up a spot and make it appear like nothing happened, so don't be disappointed if its not perfect. When I touch up a nick or something, my goal is to just make it invisible to the glancing eye. If you inspect my frame, you can see the nicks and spots where I touched up, but if you were just looking over my bike from a foot or two away, it'd be a little more difficult.