I have experimented with a lot of different methods for touching up scratches on my throws. Sharpies, paint markers, acrylic paints, enamel paints, etc… All of those methods look fine when applied, but wore off within 30 minutes of play.
The method that finally worked for me was Dupli-Color Scratch fix 2in1 car paint. I cleaned the scratch thoroughly, applied 2 thin coats, and let it sit for 24 hours. After days of play the paint still looks great. This stuff can be picked up from any AutoZone for about 7 bucks. I’ve only tested “Universal Flat Black” so I can’t vouch for any other color.