Old thread I know but only one I could find for now on this topic.
YES I have a 2014 GT CS black and YES I can see the brown primer through the thin transparent coat of paint on areas on the car in certain lighting mostly in the sun. The total car takes on a grayish tint in the sun, not a true black. I would say in general the paint coverage is about 95% and in certain areas about only 80% coverage covering the primer. IT is what it is because repaint is not a option.
Its amazing that they @#!& *# a other wise good paint job over saving a extra $50.00 in base coat material .

 

Most likely the primer (gray) but I had that on my old peweter silver metallic 00 mustang. I had noticed one spot on the hood had a weird reflection at just the right angle under the sun or fluorescent lighting shortly after I bought it new.

Because the hood was fiberglass, the coverage was inconsistent so the dealership repainted it under warranty.