Two factors. On, the different levles of brightness in colors. Even in a black and white photograph varing levels of brightness even in the same color give it contrast. Imagine a dark red square placed in the middle of a light red circle. The dog reconizes the shade and shape of the (white)bumper lying in the (white) grass. Keep in mind the bumper is more or less uniform in brightness and shape. The grass or trees are far from uniform. Second, the retina of the dog contains more rods which give it greater motion detection.They say dogs see green as white.....so why would a white bumper that is thrown across the front of green trees stand out to a dog...? and why does a dog see a white bumper at in a green grass field?