Sand the area lightly around the scratch and then apply a primer. Look around for white primer. Harder to find, but better for covering with white paint. As for the top coat itself, maybe it would be better if you'll go to a pint shop for a quick spray. White had a ton of different shades just like any other color, and even if you'll find exactly the same shade that the car was originally painted, it doesn't mean that it won't be noticeable on the car.
In our club we have a few white and red ZXs, that everyone of them is different shade! That's due to paint fading and amount of polishes it had.
As for the rust protection, if you don't see bare metal - it's fine. If you do see bare metal - the primer will protect it. Do 2 coats of primer in that case.
Good luck!