How do you know dogs are color blind?

There are two ways to research an animal's ability to see colors.

One is by dissection.

The retina in the eye contains cells that react to incoming light, and different cells react to different colors.

If an animal hasn't got the kind of cell that reacts to red colors, then that animal can't see red.

Since dogs are easy to train, one can also test their color vision experimentally by trying to train them to recognize different colors.

It's a bit tricky, since they have a very good sense of smell. If the colors were to smell differently, they might use that to figure out what they're expected to do instead.

Current research indicates that dogs aren't entirely color blind, but that they have poorer color vision than humans.