Sunlight looks white, but it contains many colors. As sunlight passes through Earth's atmosphere, molecules scatter shorter blue wavelengths more effectively than longer red wavelengths. That scattered blue light reaches our eyes from all directions, making the daytime sky appear blue.

The same principle also explains why sunsets look red and orange. When the Sun is low, its light travels through more atmosphere, scattering much of the blue away before it reaches us. Everyday beauty often begins with invisible physics.

Reference:

Wikipedia: Rayleigh scattering

https://en.wikipedia.org/wiki/Rayleigh_scattering