When you walk down the street with your dog, the almost endless array of hues you see -- from the brilliant blue sky and lush green grass to an ordinary red stop sign -- appears much differently to your pet.
Dogs have just one-tenth the concentration of color-capturing cones in the back of their eyes that humans have. So while we enjoy a smorgasbord of tinges and tones, dogs only see two colors: blue-violet and yellow, as well as any blends of these colors. The rest of their world appears in shades of gray like a dreary winter's day.
"Restrictions in color vision are probably of limited consequence in dogs, as it is likely that dogs react only to colors of biological importance to them," veterinarians Christopher Murphy and Paul Miller wrote in their 1995 study about canine vision, published in the journal of the American Veterinary Medical Association. Basically, since dogs don't forage for brightly-colored fruits like apples and oranges, there isn't a lot of value in seeing those colors, explains Miller, compared to being able to break the camouflage of prey or seeing potential threats.