Previous work on canine color vision has found that dogs do see colors and that colors are more important to them in many cases than the level of brightness of an object. However, studies of their eyes and the light sensitive cells in them have suggested that their color vision may be similar to people who have deuteranopia, better known as red-green color blindness.
In a recent study, scientists in Italy tested this hypothesis with a behavioral test. They showed dogs a series of colored targets that depict movement—in this case a red running cat against a green background—in a way that is not detectable unless the colors of red and green can be distinguished. The running cat was chosen because with a series of tests with dogs not used in the final experiment, a cat image in black and white prompted more reactions than the other images considered.
via Whisker Therapy