Why do sky appear 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.
When the sunlight passes through the atmosphere, the fine particles in the air scatter the blue light more strongly than red. The scattered blue light falls on the eyes due to which we see sky blue.
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 colour. Do you know why? The red is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance.
Why sky appear red at Sunrise and sunset ?
Light from the sun near the horizon passes through thicker layers of air and largest distance in the earth's atmosphere before reaching our eyes.
However, light from the sun overhead would travel relatively shorter distance. At noon, the sun appears white as only a little of the blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun.