Short answer
A cloud layer turns the whole sky into a large, fairly neutral diffuser. Light arrives from every direction, shadows soften, specular glare disappears and the spectrum stays close to neutral — but illuminance drops to perhaps a tenth or a hundredth of direct sun. Colours therefore read with fewer distortions but less saturation.
Under full cloud, the sun disappears as a separate source. Cloud droplets scatter all wavelengths nearly equally, so the light emerging from the base of the cloud keeps most of the sun's spectral balance while losing its direction. The result is diffuse light from the whole sky dome, brightest overhead and dimmer towards the horizon. There is no second, bluer shadow light and no warm direct beam, so the difference between lit and shaded faces of an object shrinks to a difference of brightness rather than of colour.
Reference tables put an overcast day around 1,000 lux and a very dark one around 100, against tens of thousands of lux in sunshine; thin bright overcast can sit well above that. That drop matters for colour. Perceived colourfulness rises with luminance, so the same painted door or planting scheme looks quieter and greyer under heavy cloud than in sun even though its reflectance has not changed. Near-neutrals move least in hue under overcast light, which is why it is a good time to compare them, but they also look their dullest.
Colour viewing standards simulate diffuse daylight rather than direct sun, and overcast light is the natural version of that. With no specular glare, you see body colour rather than surface sheen; with no directional beam, orientation and texture barely affect the reading; with a stable spectrum, a match that holds now will hold for most of the day. For samples that will be seen mainly outdoors — render, cladding, fencing, car paint — an overcast day is the most honest test, followed by a check in sun to see the flattering and harsh extremes.
Rooms receive a small fraction of outdoor overcast light through their windows — often a few per cent — so a room that is pleasant on a sunny day can be dim and cool on a grey one. This is the condition the site's north-facing and low-light room guides are written for: colours read flatter, pale cool neutrals go grey, and artificial light often has to be switched on during the day, mixing its spectrum with the grey daylight. Judge interior colours on an overcast day as well as a bright one; it is the room at its least flattering.
| Condition | Typical illuminance (lux) | What changes physically |
|---|---|---|
| Direct midday sun | 32,000–100,000 | Very high illuminance with a hard directional beam; shadows are lit only by bluer skylight, so lit and shaded faces of one object receive different spectra. |
| Open shade under a clear sky | 10,000–25,000 | The sun's direct beam is removed, so the light is both dimmer and markedly bluer than sunlight. |
| Overcast daylight | 100–1,000 | Light arrives from the whole sky with soft shadows; illuminance is one to two orders of magnitude below direct sun. |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
An overcast day typically provides on the order of 1,000 lux and a very dark overcast day around 100 lux, compared with tens of thousands of lux in direct sun.
Caveat: Order-of-magnitude reference values; bright overcast can be much higher.
FactStrong evidence
Cloud droplets scatter visible wavelengths nearly equally, so light transmitted through a cloud layer keeps a broadly neutral spectral balance.
Source: The Color of Clouds (JetStream); HyperPhysics (optics, atmospheric optics and vision pages)
Colourwise interpretationModerate evidence
Overcast daylight is the most reliable natural light for comparing outdoor colour samples, because it removes directional glare and the sun–shade spectral split.
Based on: Follows from the diffuse, neutral character of cloud-transmitted light and the use of diffuse daylight simulators in viewing standards.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.