Dogs see blue and yellow, and one 1930s manual said otherwise

Everything below about colour vision in dogs comes from what actually happens rather than from what is supposed to.

Where the claim came from

A widely circulated dog-care manual published in the 1930s asserted that dogs see only in shades of grey. The author was an influential writer on dogs rather than a researcher, and no evidence was offered for the statement. The book was popular and frequently reprinted, and the claim was repeated in later manuals that drew on it.

By the time anyone tested it properly, the assertion had been standard for decades in the literature owners actually read. It is a clear case of a single confident sentence in a popular book outrunning the absence of any research behind it.

What the eye contains

Dogs have two types of cone receptor, sensitive to short and medium-to-long wavelengths respectively. That arrangement supports genuine colour discrimination, but across a narrower range than three-receptor human vision provides.

The practical result is that blues and yellows are distinguishable while reds and greens tend to collapse into similar shades. This is the same broad arrangement found in most mammals, and it is the normal mammalian condition rather than a defect. Behavioural tests, in which animals choose between coloured targets for a reward, confirm what the anatomy predicts.

Where it shows up in practice

A red ball thrown onto green grass is a difficult target for a dog, because both surfaces look broadly similar to it. A blue or yellow toy stands out clearly against grass, which is why they are easier for a dog to locate visually. Owners often assume a dog that cannot find a toy is being inattentive, when the object may simply not be visible as distinct.

Checked against the record, working and assistance dog training frequently takes this into account when choosing equipment colours. It is a rare animal fact with an immediate practical application in an ordinary afternoon.

What dogs have instead

Their eyes are considerably better than ours in low light, helped by a reflective layer behind the retina that gives light a second pass. That layer is what produces the bright eyeshine in photographs and torchlight. They also detect motion more readily and have a wider field of view, at the cost of poorer detail resolution.

Their visual world is optimised for a different set of problems than ours, principally detecting movement at dawn and dusk.

Smell dominates their perception to a degree that makes visual comparisons somewhat beside the point.

Why the grey claim persisted

It fitted an existing assumption that colour vision was a special human faculty connected to higher perception. Testing animal colour vision requires careful behavioural experiments, which were not widely done when the claim took hold. The claim was also harmless in the sense that nothing obviously went wrong for anyone believing it.

Beliefs with no visible cost are corrected slowly, because nothing forces the question. The correction has spread reasonably well in recent decades, though the older version still appears regularly.

Other animals worth comparing

Cats have a similar two-receptor arrangement with their own strengths in low light and motion detection. Birds typically have four receptor types including one sensitive to ultraviolet, giving them a colour world humans cannot imagine.

Many insects also see ultraviolet, which reveals patterns on flowers that are entirely invisible to us. Assuming that human vision is the standard against which others are deficient gets almost every comparison wrong. Dogs are not colour blind so much as differently equipped, which is a more accurate and less patronising way to put it.

The takeaway

One popular manual said grey, nobody checked for decades, and dogs have been chasing invisible red balls ever since.

Ask who gained from the claim being repeated. That usually explains the rest.