It may surprise you to know that this well known statement is in fact a myth. Dogs in fact can see in color, even though the amount is minimal. Dogs can see the amount of color that a typical human with color blindness can. It is not so surprising that our vision is different from a dogs, but what is it that makes it different? Eyes seem to work the same way for everyone so why can humans see differently than dogs can? To answer this question we have to look at it from a biological stand point.
Color comes from the back of the eye from the retina. The retina contains calls called photoreceptors. The macula, which is the most sensitive part of the retina, holds millions of these photoreceptors. Cones and rods are the main two kinds of photoreceptors. There are more rods than cones. The rods are responsible for night vision and side vision. Rods are also more sensitive to light. Cones play a much larger role when it comes to our vision, and we rely on them a lot. The cones are responsible for responsible for sharp, detailed vision, and most importantly in my opinion, colored vision. Dogs also have cones and rods just like we do. Humans have 6,000,000 cones which help us perceive red, blue, green, and yellow. Dogs only have 1,200,000 cones and they can only identify blue and yellow.
How was this tested? Russian scientists from the Laboratory of Sensory Processing carried out experiments to test the theory. They started off by printing out four different colored papers: Dark yellow, dark blue, light yellow, light blue. The reason they used the different shades were to test if dogs actually used brightness to distinguish between items. The scientists proceeded to pair the dark yellow and light blue sheets together and the light yellow and dark blue sheets together. They were placed in front of two food bowls placed inside of locked boxes. After, they unlocked one of the boxes and put the dark yellow paper in front of the box which held a piece of raw meat. There were multiple trials and each one involved eight different dogs, varying of breed and size, given the opportunity to attempt to open one box before being removed. The results were that it only took the dogs three trials to learn which colored paper was in front of the box with the raw meat.
So in conclusion, dogs CAN see in color, even though this color vision is limited. Dogs only have 1,200,000 cones in their retina, as opposed to humans who carry 6,000,000. Humans can see red, blue, green and yellow, and dogs can only see blue and yellow. This is equivalent to what a human who is color blind might see.
Fun Fact: If you have ever seen any dog competitions or watched them on TV, you will notice that many of the obstacles are usually blue and yellow, now you know why!
Here is a video explaining things further