That's gonna be a toughie to find a good color to use for a border that doesn't actually look added. Since there is a piece of white already sticking out--it doesn't work, does it? At first I thought about a creamy muslin but maybe not. Toward the bottom of the picture there is a nice green in some of that. If you can find that, it might be a nice compliment to the piece. Your local quilt shop may have what they call feedsack fabrics that could compliment this one but it really needs a "stopper" border first. Isn't it fun to find someone else's work to finish and consider the one who did it? Look how well she matched seams! Great find.