Do dogs only see in black and white?

Not entirely. It is generally agreed that dogs can distinguish fewer colors than humans, but their vision isn’t entirely restricted to black and white. Rods and cones are the two types of receptors in the retina that most of us have heard of. The rods are more light sensitive and help for seeing in low light conditions. The cones help with focusing on more detailed objects and are what help distinguish between colors.

A dog’s retina has a much higher proportion of rods than cones than the human retina. This is why we can see more details at a longer distance than our dogs, but they can see better than we can in low light conditions. One estimate is that a dog can see at about 25 yards what we can see at 75 yards. If this is the case, we could call a dog’s vision 20/75 on the standard 20/20 scale. Of course these are estimates as it would be pretty difficult to get a dog to read an eye chart at any distance.

So why did the dog’s and man’s retinas evolve this way? Most likely because it was more advantageous for the dog to be able to see in low light conditions. The nearest wild relative to the dog is the wolf, which often hunts near the hours around dawn and dusk. Many predators hunker down during the middle of the day and thus don’t need to have the best vision in this time. Man evolved from any nocturnal habits such as these millions of years ago and has likewise benefited from having better daytime vision- ie., we have evolved to have more cones than rods.

Source: Wikipedia’s entry on dogs, eyesight.