It depends how deep the scratch is your talking about. Is it through the paint or not that kind of thing. If it is through the paint, or deep enough you can pick at it with your fingernail, then I would get a duplicolor 2 in 1 pen. They match well, and the pen end is great for those.
If its not, then I would try newfinish scratch remover, followed by a very light rubbing compound, glaze, and wax.
A long scratch deep enough to go down to metal....? You'll be adding so many layers of touch-up paint you'll give up before it's nearly done.
That sounds like it needs sanding, a body filler, more sanding, and a paint job. You might be able to get by with a can of spray touch-up and a clear coat, for the final painting. Hard to say....and it depends on how picky you are.