The problem with trying to touch up a marquee is the same as trying to touch up a pinball backglass. The area's that get touch up might color match well, but how translucent or opaque the touch up is will be different than the original screening. Once you light it from behind, the touch up will stick out worse than the minor scratches.
I had a Pac marquee with some big scratches in the white that I was able to reduce. first I sprayed the backside with Krylon Triple Thick, a clear coating. Once the triple thick dried, I misted a light coat of white spray paint across the back of the entire marquee, keeping an even coat. It came out pretty good and the scratches are no longer noticable unless you look really close.
If you've got scratches in colors it will be very tough to make it look nice. put a coat of triple thick on first so you can test your touch up and wipe if off before it dries if you don't like the results.
D