You can purchase color matching paint from any good Yamaha dealer that is no problem, just make sure you get the correct year range. The problem you have is that you cannot just paint the spot that is worn off if you want to blend the paint in. Plus if the lines wore the paint down to bare metal then you will need to prime before you paint.
What do you want the end result to look like? If you want it to be undetectable go to a professional and have them do the work. If you do not want to spend the couple of hundred bucks (absolute maximum) it will cost to have it done correctly then find someone that knows what they are doing to advise you on how and where to blend back to. You will also need to understand how to prepare the surfaces before priming and painting including wet sand between coats and applying clear coat once you have painted.
I have painted the lower unit on my last boat several times to a near perfect match. It is not hard to do but takes time and patience to do it right.
