Some people think dogs see the world in black and white. This is not true. Dogs can see colors, but not as many as people can. They see blue and yellow well.

But it is hard for them to tell red from green. In one test, dogs chose things by their color, not by how bright they were. So a blue toy may be easier for a dog to find on green grass.

Old books often said that dogs see the world like a black-and-white photo. However, in 1989, American researchers trained three dogs to tell colored lights apart. They found that dogs have two types of cones, the cells in the eye that sense color. Humans have three.

As a result, dogs mainly see the difference between blue and yellow, and they have trouble telling red from green. Many scientists still believed that dogs used brightness rather than color in daily life. But in 2013, Russian researchers tested eight dogs and found that they chose by color even when they could have used brightness. Color, it seems, is useful information for dogs.

The notion that dogs inhabit a black-and-white world was once common, but research has overturned it. In 1989, researchers at the University of California, Santa Barbara, trained three dogs to discriminate colored lights and inferred that the canine retina contains two types of cones, most sensitive at roughly 429 nanometers (blue-violet) and 555 nanometers (yellow-green). Dogs are therefore dichromats: they distinguish colors mainly along a blue-yellow axis, while reds, oranges, and yellow-greens probably look similar, and red and green are hard to tell apart. For years, many assumed that dogs relied on brightness rather than color in practice.

A 2013 study at the Russian Academy of Sciences challenged this. Eight untrained dogs learned to find food using cues that differed in both color and brightness; when the two cues were then set against each other, the dogs followed color. Both studies were small, however, so their conclusions should be generalized with care. A 2018 review added that dogs see better in dim light but have lower visual acuity than humans, and urged researchers to design experiments around how dogs actually perceive the world.