I SOOOOO identify with you on this. I scratched my 4 month old Vivid the exact same way,in the exact same place, only with a kitchen knife, and it bothered me so much that I gave it to my daughter (she needed a new phone anyway) and got a brand new S3 a few weeks ago.
The only way you might match the paint is to check at an auto supplies store that carries those little bottles of touch up paint. They have all colors of metallic paint. If you do this though, I recommend that you apply the tiniest bit, and if it doesnt look right, use a soft cloth to wipe it away before it sets. Good Luck.