Your on the right track, it's a matter of matching the case as closely as possible.
Search online, perhaps there is Mac case matching colors/fill for scratches.
If your scratch has pushed metal up and over the surface level of the case, sort of like what occurs with a meteor crater,
then you first need to use a metal bar and a small hammer to very, very gently tap it down so it's flush first, before using the paint/fill.
You can tell if you need to do this if you run your sentitive tip of one of your fingers gently over the scratch, you will feel it's edges are slightly raised.
Another trick is to use a new flat razor blade, and gently run it over the scratch, you will feel and see if it's not perfectly smooth on the surface.