Why Does Everything Look Grey in the Dark?

Everything looks grey in the dark because the cells that see colour switch off. Your retina has two kinds of light-sensing cell. Cones see colour but need plenty of light. Rods work in near darkness but come in only one type, so they cannot compare wavelengths and therefore cannot see colour at all. As the light fades, the cones fall silent, the rods take over, and the world you see becomes a single-channel picture of brightness: shapes and shades, no hue. You have not lost the colours. You have lost the instrument that measures them.

• Cones see colour but need bright light; rods work in the dark but come in one type and cannot see colour.
• As light fades the cones switch off and the rods take over, so the world drains to grey.
• Blue lasts longest at dusk because rods are most sensitive to blue-green light (the Purkinje shift).
• The colours are still there: a long-exposure photo by moonlight comes out in full colour.

Here is how the handover works, and the one colour that lasts longest.

Cones are the daylight cells. You have three kinds, tuned to long, medium and short wavelengths, and your brain builds every colour you see from the ratio of their responses. They are concentrated in the centre of your vision and they are relatively insensitive: they need a well-lit scene to fire strongly. Rods are the night cells. You have about twenty times more of them, they are spread across the whole retina, and each one is roughly a hundred times more sensitive to light than a cone. But there is only one kind of rod. With only one detector there is no ratio to compare, and without a ratio there is no colour. A rod can report "brighter" or "darker". It cannot report "red".

Because the handover is a crossfade, not a switch. As dusk comes on, the cones keep working but their signal gets weaker and noisier, and the rods, which were saturated and useless in full daylight, start to contribute. For a while both are active, which is called mesopic vision, and colours look washed out and slightly wrong. Eventually the cones drop below their threshold and the rods carry everything. Full dark adaptation, where your rods reach their maximum sensitivity, takes about half an hour, which is why a night sky keeps revealing more stars the longer you stand under it.

Because rods are most sensitive to blue-green light, at roughly 500 nanometres, while the cones as a group peak nearer to yellow-green. As the cones fade and the rods take over, the eye's overall sensitivity slides towards the blue end of the spectrum. Red flowers go dark and grey first; blue and blue-green ones seem to glow on for a while after. Painters have known this for centuries and it has a name, the Purkinje shift. It is also why emergency vehicles and night-time signage lean on blue, and why a red torch preserves your night vision: red light barely registers with the rods, so it does not undo the half hour of dark adaptation you have built up.

Sensitivity and colour are different jobs. Colour requires at least two detector types with different wavelength preferences, so the brain can ask "which one fired more?" Rods all contain the same light-sensitive pigment, so every rod answers the same way to a given brightness regardless of its wavelength. A dim blue and a brighter red can produce identical rod signals. The rod system chose to pool everything into one very sensitive channel, which is the right trade for seeing a predator at night and the wrong one for admiring its colour. Cats and dogs, as it happens, made a version of the same trade; the dogs and cats article explains what they see.

The wavelengths are all still there. Moonlight is reflected sunlight with the same spectrum, only about half a million times dimmer, and a red rose reflects red under the moon exactly as it does at noon. What is missing is enough photons for your cones to build a colour from. A long-exposure photograph taken by moonlight comes out in full colour, because a camera sensor can simply collect light for longer than your eye can. The greyness is a property of your eye at low light, not of the night. Colour is something a visual system does with light, and at night yours has switched to a system that does not do it.

Sit with a brightly coloured object at dusk and do not turn a light on. A CMY Cube works well because it holds cyan, magenta and yellow on one object and mixes them where the faces overlap. In daylight it is a riot of colour. As the room dims, watch the magenta and yellow faces lose their colour first, drifting towards dark grey, while the cyan face holds on longest, exactly as the Purkinje shift predicts. By the time you can barely see the cube, all three faces are the same grey and only their brightness differs. Then switch on a lamp and the colours snap back in less than a second, because cones recover from dark far faster than rods recover from light.

Because no single detector can be both sensitive enough for starlight and discriminating enough for daylight colour. Evolution solved it by fitting two. Most mammals lean on rods and get by with two cone types or fewer; primates leaned into cones and gained a third, which is why we see fruit against leaves so well and see so little at night. A cat in a dim room and a person in a bright one are both using the right tool for their light. Neither is seeing "the real colours". There are only wavelengths, and eyes that sort them.

Everything looks grey in the dark because your colour cells, the cones, need bright light and your night cells, the rods, come in only one type and cannot compare wavelengths. Colour fades gradually as one system hands over to the other, blue lasts longest, and the colours are all still there for a camera that can wait. Watch a colour cube at dusk and you can see your own eye make the switch.

Watch colour arrive and leave with the light, and see the world through a different lens.