Short answer
Cloud droplets are comparable to or larger than the wavelengths of visible light, so they scatter all colours almost equally (Mie scattering) and a sunlit cloud looks white. Clouds look grey when they are thick enough to stop most light getting through to their base, or when they are seen against a brighter background.
The molecules that make the sky blue are far smaller than a wavelength and scatter blue much more than red. Cloud droplets are a different scale: they are roughly the size of visible wavelengths or larger. Particles in that regime scatter by Mie scattering, which depends only weakly on wavelength across the visible range. Sunlight entering a cloud is scattered many times by countless droplets and emerges in all directions with its spectrum essentially unchanged. The result is white — the same reason milk, foam, fog and snow are white.
A thick cloud reflects much of the sunlight striking its top and passes little through. Its base, lit only by the small fraction that makes it through, is therefore dim, and dim white reads as grey. Towering storm clouds can be kilometres deep and nearly black underneath, while their sunlit tops remain brilliant white. There is also a perceptual component: a cloud seen against a bright sky or beside a brilliantly lit cloud is judged darker than it would be alone, because the eye judges lightness relative to its surroundings.
Because clouds are neutral scatterers, they display the colour of whatever illuminates them: gold and red at sunset, blue-grey in twilight, orange above a city at night where streetlights shine on their bases. A cloud's colour is therefore one of the most reliable indicators of the colour of the light at that height. Thin clouds and the edges of thicker ones can also show structural effects — the bright 'silver lining' where light is scattered forward at the cloud's edge, and pastel iridescence from diffraction by very uniform small droplets near the sun.
A cloud layer is a vast, neutral diffuser. Under it, light comes from the whole sky, shadows soften, and the spectrum reaching the ground stays close to neutral while its level drops. A single bright cloud beside the sun can add a large, white, secondary light source, which is why the light on a partly cloudy day changes second by second. The site's overcast light page follows those effects on how colours read; this page is about the clouds themselves.
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.
FactStrong evidence
Cloud droplets are of similar size to the wavelengths of sunlight, and the Mie scattering they produce does not strongly differentiate between wavelengths, so clouds appear white.
Source: The Color of Clouds (JetStream); HyperPhysics (optics, atmospheric optics and vision pages)
FactModerate evidence
Clouds can appear grey because they are thick enough that little light reaches their base, or because their contrast with a bright background makes them look darker.
Source: The Color of Clouds (JetStream)
Colourwise interpretationModerate evidence
Because cloud scattering is nearly wavelength-neutral, a cloud's colour is a good indicator of the colour of the light illuminating it at that height.
Based on: Follows from the near wavelength-independence of Mie scattering by cloud droplets established by the cited sources.
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.