It is not true that dogs are completely colorblind. While dogs do not have the same color vision as humans, they are able to tell yellow from blue. Like a human with red-green colorblindness, they are unable to tell the difference between red and green.
The reason for this limited range, in both the colorblind human and the dog, is that there are only two kinds of color receptors in the retinas of their eyes. While most humans have three kinds of color cells, with three different receptor molecules sensitive to blue, greenish-yellow, and red, dogs only have receptors for yellow and greenish-blue.
Canine eyes also lack another human trait: the fovea, an area especially dense with detail-sensing cells. As a result, their detail vision is not as good as ours. But they make up for this by having much better night vision and greater sensitivity to movement.
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