For much of history, it has been believed dogs' ability to differentiate between different colored objects was actually due to differences in brightness, not the actual color.
Recent research showing dogs have two types of cones in their eyes led scientists at the Institute for Information Transmission Problems of the Russia Academy of Sciences to suspect they could distinguish colors.
Humans have three kinds of cones, which allows for seeing all three primary colors.
With only two, dogs should be able to see some colors, but not others, the researchers thought -- blues, greens and yellows, for example, but not reds or oranges -- and they designed an experiment to test that.
First they trained several dogs to respond to one of four different colored pieces of paper, light or dark yellow and light or dark blue, by putting paper pairs in front of feed boxes that contained meat.
The dogs soon learned that certain colors meant a treat.
Next, the researchers placed pieces of paper with the color the dogs had been taught to respond to in front of a feed box, along with another piece of paper that was brighter, but of a different color, to see if a dog trained to respond to light blue would respond to dark blue instead of light yellow.
A majority of the dogs went for the color identifier rather than brightness identifier most of the time, the scientists said, proving they were able to distinguish color and were not relying on brightness difference to find their food treat.
The research was reported in the Proceedings of the Royal Society B.