Before the domestication of dogs, their ability to see in the dark was far more important than their ability to differentiate color, so their sight evolved differently than that of human beings. Instead of the full spectrum, a dog sees only YELLOWS, BLUES, and GRAYS. The only way this may affect your dog is if you buy him a red or green toy, because he sees them as yellows and whites, which make them incredibly difficult to find in a field of green grass, which also seems white to a dog! Next time your dog dashes right past your green tennis ball, it's not because your dog is blind -- just color blind!