One of those things that makes me just a little crazy with clients who hire me to do work on their house to get it ready to sell, is when the wife decides to show off her fashion skills and make un needed changes that only she will like. She will pick out something that she loves, but very few other people will. It might be a paint color, or a tile, or a faucet. It's just not what helps sell a property, but she wants it, so they get it and I install it, all the while knowing it was a total waste of money, and might even hurt their chances of a quick sale.
In your case, I would totally agree with your wife, but remind her that when selling the place, that more people will want to buy it for top money "WITH" the very expensive, beautiful, already finished, white fence. If that didn't work, I would get my realtor involved to go over with her what will sell the fastest, and for the most money.