QUICK SUMMARY
The sky looks blue because molecules in Earth’s atmosphere scatter the shorter blue wavelengths of sunlight more strongly than the longer red wavelengths. That scattered blue light reaches our eyes from every direction.
Sunlight contains many colors
Sunlight may look white, but it contains the full range of visible colors. Each color travels with a different wavelength. When sunlight enters the atmosphere, it interacts with gases and tiny particles in the air.
Why not violet?
Violet light has an even shorter wavelength and scatters strongly, but the Sun emits less violet light than blue, some violet is absorbed in the upper atmosphere, and human eyes are less sensitive to violet. Together, those effects make the sky appear mainly blue.
Why sunsets look red
Near sunset, sunlight travels through more atmosphere before reaching you. Much of the blue light is scattered away from the direct path, leaving more orange and red light to reach your eyes.
Source
Learn more from NASA Science — Why Is the Sky Blue?.
[…] substances absorbing and reflecting different parts of visible light—also helps explain why the sky looks blue, although the physical processes are […]