I have a black 2005 Mustang GT, and a few months ago I purchased a Zaino kit. I applied a coat of Z2 (clear coat polish). It looked great in the garage, and once I parked it outside to take a look, my paint looked exactly how you described (almost grayish). I was not a happy camper. I brought it back in the garage and applied a coat of Z5 polish (for swirl marks). After I buffed it off, I brought it back in the sun, and the paint was a nice deep black again.
I'm not saying the Zaino was the cure, but perhaps your faded color was due to the type of polish you were using. Just recently it happened to me again, but with
Meguiars Show Car Glaze. But I experimented and put it on with the PC, and it caused bad micromarring. I followed it with
Meguiars Swirl Remover and it was gone. Just a little info I thought might help you.