I just bought a spray can of Trim Paint at Autozone, mainly because of the low gloss level and color, and I painted the driver's side windshield wiper arm, even though it is not plastic, after cleaning, sanding, and priming. The old paint was showing a little bit of the metal underneath. It looks good, and should hold up in your application better than dyes, IMHO. You could also try sanding down that plastic piece and then use polishing compound on it. Maybe the oxidized layer being removed would darken the color some, but watch out for overpolishing.
Of course, the sun is the enemy of your paint and your rubber or plastic trim. I firmly believe in covering your NSX with a layered cover that breathes, Saving it now means you won't have to replace or restore it anywhere near as soon.
Regards,
Bill
Edit: Since the original post, I have polished the wind deflector
with Turtle Wax Scratch and Swirl Remover. The result is a
deeper black with a semigloss that is a good companion
to the other black pieces in the area. See covering, above.