He is bullshitting you.
A lot of people are saying that it just needs to catch up to the rest of the cars level of fade. I don't even see how that is possible? Wouldn't the whole car uniformly fade, always leaving the repaired panel darker then the rest of the car? Unless of course, you always park your car in a way that it is all in the shade except for the one area that needs to fade.
The body shop should have been able to either blend the new factory paint in with the rest of car in a convincing manner, or re-mix the factory yellow so it will look the same as the rest of the car. Having the one panel they painted not match the rest of the car is unacceptable IMO. You should take it back again and demand that they do it right.