Light comprises of 5 colours, red, orange, yellow, green, blue, whereby blue has the the shortest wavelengths. Light is able to be scattered into these colours through the molecules in the air the atmosphere. With blue light having the shortest wavelength, blue light is the colour of light reflected the most by the molecules. Thus, the sky looks blue.
Image from: http://www.sciencemadesimple.com/img_sky/bluesky.gif