There are likely some dogs who suffer from color blindness, just as some humans do. But, in general the long-held misbelief that dogs are color blind is simply not true. It is true that dogs do not see as many different colors as humans can see and the colors dogs do see are somewhat muted. But, there is a biological reason for this.
A human’s eye has 3 different kinds of special light catching photoreceptor cells called cones, each tuned to different wavelengths of light. A dog’s eye only has two kinds of cones so a dog sees mostly shades of yellow, blue and gray.