I usually use model enamel for touch-ups on my bikes and bikes I am going to sell. The basic Testors enamels work well and I can get a close match by mixing paints. Adding a little silver to a base color can match the metallics. For hard to match colors I have found a wide variety of metallics and other colors at hobby shops. I usually don't feather the edges of the chips, instead I use enough paint to fill or pool the area up to the edge to give it a smooth appearance bond to the original paint. If there is rust on the the metal I use a small screwdriver or awl to scrape to the bare metal up to a solid paint edge. I've been happy with the results and the touch-ups have been durable.