Frankly, one of the reasons I enjoy coming here to discuss all things first gen is that the threads rarely take ugly turns, as this one did.
Defining "restored" has little to do with the OP's question. It is a valid topic of conversation; just not for this thread.
The OP asked a simple question: does the color change affect value?
Yes it does. Different amounts to different people, but the color change will automatically preclude a certain percentage of potential buyers of a "numbers" car. I went through this a while back helping a friend sell his original drivetrain Z/28 that went from FG with DG interior to Dusk Blue with black interior. He still got a very good price for it, but a lot of guys were not even interested once they learned of the color change.
If the seller is willing to wait for that buyer who does not care about the color change, he or she will likely get ALMOST as much as if it were an original color.
At least the OP was advised to get the thing checked out. I would go one further: post clear pics of the tag, and all the stampings. If they are obviously bogus, then the OP saves a boat load of money on the personal inspection by Jerry M. If they look good, then go for the full inspection. One can't be too careful. I would guess about 1/2 of the 69 Z/28s advertised, are not what they are represented to be.