I imagine it would be very difficult to get the dog to sit still at the eye doctor long enough to find out. So how do we know that dogs are colorblind?

To begin with, they aren’t. they don’t see as many colors as humans, but they do see more than black and white. Dogs have two types of cells on their retina that recognize wavelengths of light, called cones. Humans have three types of cones, so we see more colors. It is likely that dogs confuse reds and greens. This type of vision is similar to humans who are red-green colorblind.

How do we know?

Scientists taught dogs to recognize colors, but they often confused reds and greens. Beams of light were also flashed into dogs’ eyes and the pattern of light that was reflected back was analyzed. These results were then compared with the pattern produced when the same lights were flashed into human eyes.