First keep in mind touch up will never color match. It will definitely improve the look but if someone get close to the paint, it would be easily noticeable.
The easiest way is to clean the scratch with cotton bud and paint thinner. Then use Soft99 Kizu Pen to fill the scratch.
This pen is like crayon color pen for kids which makes it super easy to use. You just need to gently fill the paint using the pen, don't push it hard.
When dried, you can even compound or polish it to improve its look. Check the video for demonstration. The video will start playing from 11:53 where the Kizu Pen is used.
