Normally when I've done touch ups of small areas on a bike using an airbrush, I've still got some of the original paint available and so can obtain a perfect match. The best approach to getting the best match in your case is to take a sample of the paint to an auto paint supplier who will be able to closely match the colour for you. That said, you'll still see the difference in colours if you re-spray small areas of the tank but, if you re-spray the whole tank, the slight difference in colour with the rest of the bike may not be noticeable. This is the same approach as with a car where you normally have to re-spray at panel level.