Depends on the depth of the scratch. If the scratch is white or another non-metal colour, then it probably has not broken through to the metal. This makes it a bit more straightforward.
If you can see bare metal, then it is paintable, but you'll be applying what is effectively topcoat to bare metal and this isn't great - particularly if the scratch has been there for some days. The oxidisation process that creates rust may have started. In that instance, touching up the scratch is a very temporary measure as it might start looking dreadful within weeks.
The touch up pens aren't a bad job, tho I recommend getting a brush the same and using them both. There may be parts of your scratch that are finer than the point of the touch up pen and the last thing you need is to put a big glob of paint on when you could have hidden part of it much more effectively with the fine brush.
Try using it on a piece of spare metal (or a bit of paper at least) to see how it works first so you don't end up putting too much on by accident. I think you could probably put a few drops into a clean jam jar lid or whatever for use with the brush on the more fiddly bits.
Touching up on unbroken paint might hide the scratch so well that it may be years before you decide to repair it properly. This being the case, you would want to do the best job you can. Keep the paint as well for other parts of the car that get the odd dig.