I'm planning to try the Ford OEM Louvers in their stock black primer and see how that looks with my Sterling Grey car.
If the stock primer finish isn't quite up to snuff, I might shoot a quick coat of matte black on them.
And, if after a while the combo just doesn't seem to work, I'll strip them and get them painted to match the car.
One important point: If you take the plain/primered louvers out into the weather strapped to your car, they will NEED to be stripped/sanded/prepped/whatever and reprimed before painting, should you ever decide to paint them. The primer coat that comes on them, or the plain plastic if you get some unprimed, will pick up all kinds of environmental 'gunk' that needs to be cleaned off and a fresh primer coat applied before any painting with finish colors.
FYI, if you do a Google Image search for 2013 Ford Mustang OEM Louvers you'll see some examples where people are running either unpainted, or black louvers on non-black cars. I think it looks pretty good on some colors, not so hot on others.