The sky appears blue because blue light is scattered more than other colors by the tiny gas molecules in the atmosphere.

Sunlight contains all colors, but when it enters Earth's atmosphere, it hits tiny air molecules that scatter light. Blue light waves are shorter and scatter more easily than longer wavelengths like red. This scattering sends blue light in all directions, filling the sky with blue during the day. Even though violet light scatters too, our eyes are more sensitive to blue and there's less violet in sunlight, so we see a blue sky instead of purple.