Dogs have only two kinds of cones in their retinas, whereas humans have three. As the video above explains, the lack of a third kind of cone does not mean that dogs see in only black and white. They can also register yellow and blue, but not red. Dogs can also distinguish between different shades of yellow, blue, and grey.
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Thinking about Professor Conway’s suggestion that color vision is useful to us, because it allows us to notice changes in skin color caused by sickness (or injury or other medical problems), I’ve compiled a short list of some sicknesses that noticeably affect skin color.
-jaundice (causes yellow skin and eyes). This condition is common in newborns.
-carotenemia (causes orange skin). This condition is considered harmless, but it can be mistaken for jaundice.
-cyanosis (causes blue skin). Indicates decreased oxygen in the blood.
-vitiligo (causes blanched skin due to destruction of melanin-producing cells).