The problem is not only with the White Balance. In fact, it is a logical enhancing not much different from that of Photoshop and which imitates the choice
of an appropriate material in the traditional process (film, paper). To my taste, the daylight is the best option, especially, if you are not in harry and want to enjoy taking photos and to get the best one.
Another problem is the background. A smooth silver surface (or luster bronze) acts like a number of tiny mirrors reflecting the blue or gray sky, walls, the shirt of the photograther and so on. Worse of all, they reflect the body of the camera and its interior. This may lead to agly black spots.
I am trying to circumvent this by using the zoom and taking shot in
oblique directions against a suitable background (e.g., a blue sky).
Unfortunately, it is not so easy to keep the coin in the focus.
I use the Photoshop enhancing without hesitation (it is like a choice
of a contrast paper). You can see some results of my experients on this board;
Of course, my advices are not professional.
A good camera seems to be a necessary condition for success
good coins. I still could not find a solution for medium quality
the coins are quite nice in hand but not on photos. Impossible to get satisfactory ones? I do not think so...