Sunlight looks white, but it is a mix of wavelengths. Passing through the atmosphere, it interacts with nitrogen and oxygen molecules. For particles much smaller than the wavelength of light, Rayleigh scattering sends short wavelengths, especially blue, in all directions. That is why the daytime vault looks blue.

Violet is scattered even more strongly, yet the Sun emits less violet than blue, some is absorbed higher up, and the human eye is less sensitive to violet. What we perceive therefore stays blue, not violet.

At sunset the Sun sits near the horizon. The path through air lengthens. Much of the blue is stripped from the line of sight, so red, orange and yellow remain between the eye and the disc. Same sun, same atmosphere, different optical path.

Dust, smoke, pollution and fine droplets do not paint a sunset by themselves, but they change absorption and scattering. Two evenings with the Sun in the same position can look very different.

On the Moon, without a comparable atmosphere, the sky stays black even when the Sun is visible. The blue belongs neither to space nor to the solar disc: it is an effect of light travelling through Earth’s air.

The account describes a relatively clean atmosphere. Clouds, aerosols and haze can shift the hues without cancelling the Rayleigh mechanism.

Image: Mr BondMax, colourful sky over the Maldives. Wikimedia Commons, CC BY-SA 4.0. Cropped to 16:9. Licence. Original file.

Sources consulted: NASA Space Place — Why Is the Sky Blue?; Encyclopaedia Britannica — Rayleigh scattering.