The reason that the sky on earth is blue in color is because of the blue light in the atmosphere.
What makes the sky blue is the blue light that is scattered by tiny molecules in all directions of the earth's atmosphere.
The blue light and blue color is scattered about in the earths atmosphere more than other colors in the earths atmosphere because it travels as shorter, smaller waves.
The sky can turn other colors at times as well when the other colors such as green, gray etc become scattered through the earths atmosphere.
However the blue light is what is scattered more in the earths atmosphere which is why the sky is blue most of the time.
When the sky changes color such as to green or gray then it usually means either a thunderstorm is on the way or rain is about to happen.
Or in winter ice or snow could come down later.