The daytime sky looks blue because air molecules scatter shorter wavelengths of sunlight more strongly than longer ones. Blue light reaches our eyes from many directions across the sky. NASA's explanation of sky color starts with two ingredients: sunlight contains many colors, and the atmosphere redirects them unequally.
White sunlight contains more than one color
Light that appears white contains a range of visible wavelengths. Blue has a shorter wavelength than red. When sunlight encounters the gases in Earth's atmosphere, some of that light is scattered rather than continuing along its original path.
That changes the question from “What color is the air?” to “Which light is arriving from that direction?” The sky is not a painted ceiling. Its appearance is the result of light traveling through it.
Why isn't the sky violet?
Violet is also strongly scattered. The National Weather Service explains that our eyes are more sensitive to blue and sunlight contains more blue than violet energy. Scattering alone is therefore not the whole explanation of the color we perceive.
The molecular scattering process is called Rayleigh scattering. Learning the name can help you find more detailed explanations, but the useful idea is the preference: different wavelengths are not redirected equally.
Why does the horizon look paler?
NASA describes how light near the horizon passes through more air and undergoes additional scattering. The result can look less intensely blue and more whitish than the sky overhead.
A simple observation exercise is to compare a patch of clear sky overhead with a patch near the horizon, keeping your gaze away from the Sun. Record whether they look equally saturated. This is a suggested comparison, not a guaranteed result: clouds and other atmospheric conditions can complicate the view.
A useful distinction to remember
Looking at the sky means looking at light scattered toward you. That is a different light path from sunlight traveling toward you from the Sun's direction. Keeping those two paths separate helps explain why blue daytime skies and warm-colored sunsets belong to the same story.
Keep exploring: Why sunset colors change.
