The sky isn't actually blue ,it just appears that way. The sky, after all, is just transparent air, "a stage upon where all colors dance." On a clear sunny day, we see the sky as blue. There are many misconceptions and incorrect answers bandied about — because it reflects the ocean; because oxygen is a blue-colored gas; because sunlight has a blue tint — while the right answer is often thoroughly overlooked.
According to various studies the blue colour appears because of the nature of light, atoms and molecules and some quirky parts of earth's atmosphere.
As sunlight enters the earth's atmosphere, the different colored wavelengths making up that light start bumping up against the air and "dancing" differently. The light is really a combination of ultraviolet visible light (i.e. ROYGBIV or rainbow color order) and infrared light. But as, air molecules scatter more of the shorter wavelengths in visible light like violet and blue, which give the sky its perceived color.
The process of blue light scattering more than other colours is called Rayleigh. However, during sunset the sunlight has to pass through more air scattering almost all the blue light thus we see red Sun with more bluer sky.
Colour of Sky during night-time
When you look at the sky at night it look dark, however it isn't complete black. And the twinkle that you see is not just stars, but also night sky that glows. The glow is not because of light pollution, but the atmosphere glowing naturally.
While, the glow you see on an a dark moonless night with trees and hills silhouetted against it is called airglow. This glow is because of atoms and molecules present in the atmosphere releasing a energy called quanta in the form of light.
In visible light, oxygen produces green and red light, hydroxyl (OH) molecules produce red light, and sodium produces a sickly yellow. Nitrogen, while far more abundant in the air than sodium, does not contribute much to airglow.
