I've used a black Sharpie permanent marker for effective touch-up of black metal cases. Front panels are usually trickier, but if you match color and texture very carefully beforehand, you should be able to find something that works OK in your local Home Depot, if you are very careful.
And no, like volume knobs, some brands of touch-up paint do not sound better than others, thank God!
And no, like volume knobs, some brands of touch-up paint do not sound better than others, thank God!