I just took a quick gander at an old Apple iPhone Service Guide that was sent to me, along with a replacement phone, when I called with problems recently. I never noticed before, but the staples on the white booklet are also white.
Who cares about details like this? Apple, that’s who. Despite receiving tons of multipage white documents in my life I don’t think I’ve ever seen a white staple in my life and I doubt I ever will again unless it’s on a document that comes from Apple.