The sky appears to us normally as a clear, blue color because of the random scattering of sunlight as it enters the Earth's atmosphere. Actually, the sky can be lots of different colrs, ranging from red to orange, to yellow, to blue. Sunlight coming into the Earth's atmosphere from directly overhead has the least resistance to encounter when coming in, so the shorter wavelengths get through with little or no scattering. This is what humans perceive as a blue color. Sunlight coming in parallel, which is what occurs at sunset, has to go through much more air molecules because of the angle it is entering the atmosphere. The chances of random scattering are greatly increased. This is why sunsets often appear reddish-orange, because the longer wavelengths are the only ones to make it through.
The colour of sky seems blue due to the colour of ozone gas which has blue colour.