Walnut will fade a bit, from the deep chocolate brown color when it's freshly varnished to a dark reddish brown, probably close to a darker mahogany color. Hit a couple antique stores if you're curious, they will probably have several old pieces in walnut. Seems to vary from piece to piece, I'm no expert but I would guess it's dependent on how much light exposure over the years.What will black walnut do? I bought some boards and might use them in a canoe.
Given that most canoes will spend most of the time stored indoors, protected with UV resistant varnish I don't think it's a big deal. My walnut stems look the same color they did when I built it 10 yrs ago.
Steve