Thanks everyone.
Yesterday evening I tried one of these pens
http://www.faber-castell.com.au/5575...cv2_index.aspx (which I had at home) and it is not that bad... I wanted to see how it looks in the sunlight and I cannot say I am really happy but at first sight one doesn't notice so much. Knowing where it is and looking closer you can still see it but all in all it is not that bad.
I will look into the automotive paint though. Maybe they have a colour that matches my scope 100%. It is metallic blue, and the pen I had is normal blue, that's why it cannot hide the scratch 100%.