Touch up works best on a chip. But it can't mask everything. There are lots of video on you tube with varying results. Best to use a toothpick.
If you have a scratch on the bumper it may be worth getting the entire bumper repainted. It won't cost much and all stone chips will be removed in process. Don't get some smart repair in your driveway. Go to a bodyshop. It will cost the same, but will be 10x better. My local bodyshop usually charged Around £150-200 for a bumper.
E90 330i SE Auto Graphite Grey Metalic