Veterinarians used to believe that dogs saw only in black and white, but recent studies suggest that they actually do have some colour vision—but it's not as bright as a human's. As it turns out, dogs have only 20% of the cone photoreceptor cells—the part of the eye that controls the perception of colour—that humans have.

While we can't ask dogs to read an eye chart or pick out colours, behavioural tests suggest that dogs see in shades of yellow and blue and lack the ability to see the range of colours from green to red. In other words, dogs see the colours of the world as basically yellow, blue, and grey.

One amusing fact is that the most popular colours for dog toys today are red or safety orange. The problem, of course, is that red is difficult for dogs to see and may appear to them as a very dark brownish grey or even black. So if your dog runs right past the toy that you tossed, he may not be stubborn. He's probably just having a hard time discriminating it from the green grass of your lawn.

A dog's visual acuity is also less developed than ours. Some experts believe that dogs only have 20–40% of our visual acuity. That means an object a human can see clearly may appear blurry to a dog looking at it from the same distance. Canines, on the other hand, are able to see in much dimmer light and can detect motion more easily than we can, something that served them well in the wild as nocturnal hunters of camouflaged prey.