The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.
The sky is seen as blue because of the visible wavelengths in the sky. Color blue is the weaker color and closer to the human eye and also scattered more than other colors. That is why when the sun reaches the earth surface, the eye will first see the blue color. You can check out http://spaceplace.nasa.gov/blue-sky/ for more information.