Can Dogs and Cats See Colour? What They Actually See

Dogs and cats can see colour, but they see far less of it than we do. Both have two kinds of colour-sensing cone cells where we have three, so their world runs from blue through grey to yellow. Red, orange and green all collapse into the same dull yellowish-brown, which is why a red ball on green grass is nearly invisible to a dog and a blue one is easy to spot. Neither animal sees in black and white. They see a real, if narrower, palette, and they see it best in dim light, where we start to fail.

• Dogs and cats can see colour, but only blues and yellows.
• Both have two cone types instead of our three, so red and green collapse into one dull tone.
• A red ball on green grass is nearly invisible to a dog; a blue or yellow one stands out.
• They see movement and darkness far better than we do.

Here is what that world looks like and how to work with it.

Blues and yellows, and everything in between as shades of grey. A dog's two cone types peak at roughly the blue-violet end and the yellow-green part of the spectrum. Anything that reflects mostly long wavelengths, which is to say red and orange, produces almost no signal in the blue cone and a moderate one in the yellow cone, so it registers as a dim yellow or brownish grey. Green does much the same. The result is close to what a human with red-green colour blindness sees, a comparison the colour blindness article explains in more detail.

Much the same as dogs, with a slightly stronger case for a little green sensitivity. Cats also have two main cone types, tuned to blue-violet and yellow-green, and their colour vision is generally described as more muted than a dog's, with less saturation across the board. What cats have instead is exceptional low-light vision: a much higher proportion of rod cells, a reflective layer behind the retina that gives light a second pass, and pupils that open enormously. A cat trades colour for the ability to hunt at dusk, and by that trade it comes out well ahead.

No. That idea persisted for most of the twentieth century and it is simply wrong. Dogs have two functioning cone types, they can be trained to distinguish blue from yellow reliably, and they respond to colour differences in toys and objects. What they cannot do is tell red from green, or pick a red object out of green surroundings. Two cones make a colour world; it is just a narrower one than three.

Because to a dog, red and green are the same colour. Both land in the same muddy yellow-brown region of a two-cone system, so a red ball on a green lawn has almost no colour contrast at all. The dog is finding it by brightness, movement and smell, not by colour. Swap the red ball for a blue one and it stands out sharply, because blue is one of the two colours a dog's cones are actually tuned to. A yellow ball works too. If you have ever wondered why a dog runs straight past a red toy lying in plain sight, this is why.

Blue first, then yellow. Both are colours the animal can see clearly and both contrast well with the greens and browns of most outdoor settings. Avoid red, orange and green for anything you want your pet to find by sight. This is also why so many agility obstacles and professional training tools are blue and yellow: the designers are working with the animal's eyes rather than the owner's.

They see less colour and far more motion and dark. Both have far more rod cells than we do, which are the light-sensitive cells that work in dim conditions and detect movement but carry no colour information. A dog can see a small movement at a distance that a human would miss, and a cat can navigate a room by starlight that leaves us blind. They also have a wider field of view. What they lose in colour they gain in everything a hunter or a scavenger at dusk actually needs. Our three-cone colour vision is, in evolutionary terms, a fruit-finding adaptation. Theirs is a twilight adaptation.

Many birds, most reptiles and a lot of insects. Birds typically have four cone types, including one tuned to ultraviolet, so a plain-looking bird may carry patterns its mate can see and we cannot. Bees see ultraviolet and use it to read landing guides on flowers. The record holder is the mantis shrimp, with a dozen or more photoreceptor types, though it seems to use them for speed rather than subtlety. Two cones, three cones and four cones are all just different slices of the same spectrum. None of them sees "the real colours". There are no real colours, only wavelengths and the eyes that sort them.

Take away red. A two-cone world is what you get when the red-green distinction is removed, so a strong cyan filter, which blocks red light, gets you surprisingly close. Look through the cyan face of a CMY Cube at a red ball on a lawn and watch it lose its advantage over the grass. Then look through the yellow face, which blocks blue, and watch a blue toy vanish the way a red one does for your dog. The cube's three filters are a small, tidy way to explore what any eye is and is not tuned to, which is what the science page is about.

Dogs and cats see colour, in blues and yellows, with red and green collapsed into one dull tone, because they have two cone types to our three. They are not colour-blind and they do not see in black and white. Buy blue and yellow toys, expect red ones to vanish in grass, and remember that they see movement and darkness far better than you do. Every eye is a different slice of the same light.

See the world through fewer colours, and view light through a different lens.