Dogs have a different percentage of rods and cones in the eye than we do. Also, in contrast to our three kinds of color receptors in the retina, they have only two, which results in something like our color blindness. Contrary to what scientists once thought, the world of dogs isn't quite black and white. They can perceive the groups of yellow to green and red to orange and blue to purple, but they can't differentiate between the members of each group. Thanks to the differences between their eyes and ours, they can detect a distant squirrel's movements before you can, but sometimes they have trouble locating a neon yellow tennis ball that you just threw into green grass not 20 feet away. Yelling "It's over there, you moron!" will not improve their color perception.
From In the Womb: Animals by Michael Sims, (National Geographic Society, 2009) p.37
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