The primary color of the sky on a clear day is blue. This phenomenon occurs due to a process called Rayleigh scattering, where shorter blue wavelengths of sunlight are scattered in all directions by the gases and particles in Earth's atmosphere. As sunlight passes through the atmosphere, the blue light is scattered more than other colors, making the sky appear blue to the human eye. The intensity of the blue can vary depending on factors such as time of day, atmospheric conditions, and pollution levels. During sunrise or sunset, the sky can also display a range of colors due to differing light scattering effects.