Why do people think cats and dogs are colorblind?

Answer 1

It has only been recently discovered that many animals, not just cats and dogs, can see in colour. Before, it was thought that all animals were colourblind.

Dogs, for instance, can pick up yellow and blue-purple colours as they have two cone types which allows them to see in limited colour. Cats, can also see some colours: They can tell the difference between red, blue and yellow, and between red and green, although these are less distinguishable. Cats are able to distinguish between blues and violets better than between colours near the red end of the spectrum.

There are only a few animals that cannot see in any colour at all: Seals, sea lions, walrus' and animals from the Cetacea family (whales, dolphins and porpises) have only one cone type (which is called Monochromacy).