Colors of bumpers are wide and varied. For marked retrieves, you DO want the dog to see the bumper. For blind retrieves, you don't want the dog to see the bumper until he "steps" on it. On a "mark", if the background the dog sees is very dark, use a white bumper and not a black one. If the background is mostly a whitish sky, use a black bumper or one that is half-white and half-black. If you are running the dog on a "blind", use an orange one --- you get the picture.

As far as what colors dogs can and can't see, go to www.nahranews.com/html/nahra/vision.htm