Touch up pens will not blend into the colour of the car. They will take your eye off of the nasty scratch, but will still look ugly and a bodge.
All paint that goes onto a car is "handmixed", whether it's a hand programming the computers in the factory with the correct ratios (which are often well out hence many colour codes having variants "light/dark shade" from factory) down to the guy in the mixing room measuring the paint out on the scales.
If you care about the car, get it painted properly, by a professional, in a good bodyshop. Don't bother using guys in vans that come to yours and blow it in, badly, with rubbish paint that looks just as bad as having the scratch in the first place!
If you're not fussed, leave it. It won't rust, you may get paint peeling as water gets under the lacquer.