I also have no idea what he is checking, but I would trust his experience of that breed over my non-experience, especially since he offered to exchange if he is wrong!
The color difference is probably just individual variation. Some just are a bit different shade of gold than others.
You could have two males, two females, or one of each. And if they are one of each, I don't know which is which. You're right that wing sexing isn't going to work.