Buy some silver paint, and a fine paint brush.
Dip the brush in the paint.
Wipe the brush on a piece of paper until all the paint is off.
The very lightly, brush over scratch.
Repeat until happy. 🙂
For more info, google dry brushing.
*I once did this to a macbook alu I listed on ebay as immaculate and the buyer didn't notice 😀