People have brought this up in other threads and it works pretty well for most colors. Testor's Model Enamels come in numerous shades and hues and usually you can get a pretty close match.

- clean and degrease all around the area you will be working. PrepSol or similar paint prep cleaner/solvent will remove any wax or silicone from previous polishings which can cause "fish eyes" in new paint.
- remove any loose paint
- using a rubber auto body sanding pad (3M makes a 3 pack for about $2) and some 800 grit or finer sandpaper carefully feather the edges of the damage and get the exposed metal clean
- wipe with PrepSol or similar cleaner
- using a brush, cover exposed metal with a coat of rust inhibiting primer and allow to dry for 24 hours (get yourself a couple of decent soft artists brushes, don't use the 39-cent plastic brushes sold in hardware and hobby stores)
- sand the surface lightly with 1,000 grit sandpaper on the rubber backing pad just until smooth and satiny
- wipe the surface with a clean piece of cotton t-shirt or microfiber cloth to get rid of any dust
- paint a coat of your matching enamel over the primer and a couple of mm onto the surrounding paint. Don't overwork the paint by repeatedly brushing over it. Try to get it on in a thin consistent layer in just a few strokes. Then feather the edge to leave as small a ridge as possible. If you have an air brush, you can use that at this stage and give it a few coats.
- This is important, allow the paint to dry 24 hours before using the bike but do not do anything to smooth or blend it for at least a week. Two weeks is better if the weather is cool and moist. You can ride the bike during this time but let the paint completely cure, which is different than just drying to the touch.
- After plenty of curing time, take 1,000 or 1,200 grit sandpaper and the rubber backing pad, slowly and gently smooth any minor imprefections and feather the edge into the surrounding paint. Don't overdo it, the paint layer is very thin and you need to leave enough for the polishing step that is next.
- Use an automotive polish which is a wax/silicone with a micro fine abrasive (not rubbing compound which is more aggressive) and a microfiber cloth or piece of cotton t-shirt and gently buff the area over and around the repair finishing by giving the entire frame tube a light buff, then wipe it down with a clean cloth and give the entire frame a wax with a regular wax/silicone that has no abrasive quality.

If you're like me, you'll still see the repair and feel like you could do better, but the vast majority of other people will never spot it even when you point it out. By the time the finish ages a bit and goes through a few washings, the repair should be undetectable when standing beside the bike. In any case, even if the paint is not an absolutely perfect match, the frame will be protected and it will look a heck of a lot better than a big scratch.