The sky has no true colour. While most of the time it is blue, sometimes it is not. The sky looks blue but really it is made up of all the colours of the rainbow. Each of these rainbow colours has a different wavelength. Some of these are smoother while others are choppy. Blue light waves travel in short, choppy waves. Like each of the other colours, blue light waves are scattered and reflected as they enter Earth's atmosphere and collide with gases and other particles. Because the colour blue has the shortest wavelength, it collides with nearly everything in its path and is scattered about the sky. This is why the sky appears blue. The sky can often be pale blue, gray, or even white. The reason for this is pollution. Some of the example and cause of different colours of sky are as follows.
Deep blue sky :
This colour of sky means the sky is very clean. This often occurs when a cold front brings clean air from the north, or when clean air from the ocean moves onto land.
Medium blue sky :
This colour of sky means there is lots of water vapor in the sky. It can also suggest the presence of sulfur from coal-burning operations. It may be caused by the chemical emissions of plants and trees.
Pale or milky-white sky :
This colour of sky indicates considerable air pollution from coal-burning power plants or chemical power plants. This condition often occurs in the summer when the air is still. There are also natural causes, such as volcanic activity or ocean plankton etc.
Gray or dark gray :
This colour of sky cause of the smoke from forest fires or agricultural burns.
Brown or brownish orange :
Emissions from cars and trucks can cause a layer of this colour to form over the horizon. The main component of this kind of pollution is nitrogen dioxide.