Good question man!...I gots me a 99 M750 Dark...and I would love to know the answer to that question.
I got a few nicks/rubs/wear on my macheen, no dents...would love to know how to touch it up...I have even thought of using a sharpie pen for now. I am afraid if I touch it up it will be a raised bump...I could color sand to smooth things out, but ..it's matte black..I cant sand or polish...hmmm..
A grey or black permanent pen might just be enough to keep the white below from standing out...hmmmm
plasti-kote GM 7179 from Pep Boys is a 99% match for my '02 Dark. I shot my SBK FR fender with it then clearcoated it with Krylon Satin matte. looks great and for under 10 clams.
Itmay take a little practice with the Satin to get the right effect. I shot one thin coat, let it dry for 5 min, shot a heavier coat, let it dry for 30 min, then stood back 3ft and shot "dust" at it till the flat was right. It's easier than it sounds but try it on some scrap till you get it.
Happy painting ;D