Slate grey is no more difficult to repair than any other metallic paint and any good body shop should be able to match it up perfectly. I would however take it to a body shop that is familiar with high end cars like Porsche, Mercedes, BMW, Jaguar etc.

When I took delivery of my Slate grey 997S there was a scratch about 2 inches long and fairly deep, well through the paint and into the plastic on the rear bumper that needed repair.

I had waited 6 long months for this car and all I wanted to do was drive! The repair could wait so a few weeks later I took it to the body shop of my choice where they repaired it to perfection (and the dealer paid the tab) and I challenge anyone to find the repair, I'll even point out the area where it was done! They only paint the area around the scratch and then clear coat the entire section (in my case just the bumper, in your case just the hood).

I would be willing to bet that when they are finished you too will not be able to see where the work was done. Good luck!