It's easy to see that bright red frisbee gliding across the lawn. Well, maybe for you, but not so much for Dexter. Your pup isn't colorblind, but he does have limited color vision because of the cones in his eyes. They work together to color the world in sepia tones instead of full color.
It's Not Black and White
It's an enduring myth that a dog views the world much as you see an old black-and-white movie. The truth is, though he'll never be able to appreciate the beautiful colors of a rainbow, Dexter does see some color. What he sees is limited, however, because of the composition of his eyes.
The ability to see color depends on the cones -- basically color receptors -- in our eyes. Cones detect color by responding to different wavelengths of light, which present as color. Humans who have full color vision have three types of cones, allowing them to see the spectrum of light created by the three primary colors of red, yellow and blue. Your pup gets by with two types of cones, the color receptors that pick up wavelengths presenting as yellow and blue.
Seeing What He Sees
If you have green-red colorblindness, you can get an idea of what your pup sees because you're dealing with the same cones Dexter has. However, if your color vision is fine you have to use your imagination to see life as a dog. The spectrum you see as red, orange, yellow, green, blue and violet is grayish brown, dark yellow, light yellow, grayish yellow, light blue and dark blue to Dexter. It's all variations of yellow and blue to him. As a result, he can't discern as many shades of gray as you can.
Fortunately, Dexter's eyesight isn't his most finely developed sense. His eyes' rods help him see in dim light, helpful at night. Like all dogs, he's a bit nearsighted but he's great at discerning motion at a distance. Contrast and movement are what he relies on most when he's looking for an object. If you want to play a game of fetch with Dexter, pick another color instead of red for the frisbee to chase because he'll quickly lose sight of it; go for a bright yellow or blue instead.