The sunlight while traveling through the atmosphere is broken up and scattered by the tiny particles of air, water vapor and dust. It makes the sky look blue. As we know, sunlight is made up of seven colours and it travels in waves, Light of different colours move in waves of different lengths. The waves of blue are much shorter than the waves of red or orange light.
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As the sunlight travels through the atmosphere, the waves of blue light are just of the right size (shorter waves) to be broken and scattered by the air , water vapor and dust particles, and then they are spread in all directions, while the waves of red or orange lights pass right thought the atmosphere and reach our eyes. The scattering of blue light makes the sky appear blue.
However, in the morning and in the evening, i.e. the time of sunrise and sunset, the light from the Sun passes through greater layers of atmosphere that scatter the orange and the red light waves as well. Thus, the sky appears to be of a red or an orange colour at those times.
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In Other Words
The sky appears blue because of the scattering of the sunlight by the atmosphere of the Earth. When sunlight reaches Earth, it hits suspended particles of air and dust and gets scattered in all direction. Sunlight comprises light waves of varying wavelengths each of which is seen as a different colour. The size of the scattering molecules decides which color is scattered most. So during the day the sky appears blue because more blue light is deflected towards the Earth.