The colour on the watch is not a paint, but oxidising or bluing. So if you scratched it, it is imposible to restore this, unless you will take it apart and repeat all the oxidation process. On other hand there are ways to cover that up. I'd recommend to try black nail polish or, so called, gel polish (requires UV lamp). They are available in gloss and matt. You probably would need to restore it from time to time, but it will last longer than any conventional paint.