Why is the color of the clear Sky Blue?

The molecules of air and other fine particles in the atmosphere have size
smaller than the wavelength of visible light. These are more effective in
scattering light of shorter wavelengths at the blue end than light of longer
wavelengths at the red end. The red light has a wavelength about 1.8
times greater than blue light. Thus, when sunlight passes through the
atmosphere, the fine particles in air scatter the blue color (shorter
wavelengths) more strongly than red. The scattered blue light enters
our eyes. If the earth had no atmosphere, there would not have been
any scattering. Then, the sky would have looked dark. The sky appears
dark to passengers flying at very high altitudes, as scattering is not
prominent at such heights.
You might have observed that ‘danger’ signal lights are red in color.
Do you know why? The red is least scattered by fog or smoke. Therefore,
it can be seen in the same color at a distance.