For years, if not decades, it has been accepted that dogs see the world in black, white, and varying shades of gray. According to new studies, that isn't the case. While it is true that dogs cannot see the same colors as humans do, they are most definitely not blind to all the colors on the spectrum.
For the most part, dogs' eyes are very similar to human eyes in structure and function. Eyes detect colors through light receptors in the retina called cones. When the cones are stimulated, they transmit that signal (color) to the brain. Rods, also in the retina, respond to shades of black and white and are sensitive to changes in light.
Humans and dogs have both rods and cones, but not the same amount of each. Dogs have many more rods than humans do, causing them to see far better in low light, which is great for hunting prey in the nighttime. Cones control color perception and allow the differentiation of different colors. Humans, however, are trichromatic, meaning they have three cones in the eyes that allow us to see all the colors of the rainbow. Dogs are dichromatic, so they are limited to two cones, which reduce the number of pigments they perceive.
Humans can see violet, blue, blue-green, green, yellow, orange, and red. Dogs can see shades of blue yellow, and gray. In fact, dogs' vision can be compared to a person suffering from red-green color blindness (deuteranopia). Red, yellow, or green objects are perceived as yellow. Blue and purples objects are perceived at blue, and cyan or magenta objects are perceived as gray.
Humans have something called a fovea in the eyes that allows us to see sharpened details. Dogs perceive the world as softer and blurrier than we do because of the lack of fovea. For example, a humans' best vision is 20/20, and dogs are thought to have 20/75. However, the lack of fovea allows dogs to perceive motion and movement exceedingly better than humans do.