What color is the sky? 

On a sunny day, the sky looks blue. Sometimes, early in the morning and just before sunset, it may look greenish-yellow, or even orange-red.
The sky actually has no color at all. It is a great ocean of air, called the atmosphere, which is made up of colorless gases.
The sky looks colored to us because of what happens to the sunlight which passes through the atmosphere.
All kinds of particles, such as dust, smoke, and ash, float in the air. Many of these particles come from human activities, such as burning fuels to provide energy. Other particles are the result of natural happenings. Sometimes, strong winds whip up particles of desert sand. When volcanoes erupt, they send clouds of smoke and ash into the air.
When light strikes the tiny particles that are found in air and water, the light is scattered in all directions. When the sun is overhead and high in the sky, its light is scattered by the particles in the atmosphere. The light photons with the highest energy—violet and blue—are scattered more easily than those with a lower energy—red and orange. It is this scattered color that makes the sky appear to be blue.
At sunrise and sunset, we see the sun through a greater thickness of air because it is low in the sky. Therefore, we are looking at it across the earth's surface rather than directly up into the sky. At these times of day, the sky often turns an orange-red color. This is because the photons of red and orange light are scattered through the atmosphere around us.

As the sun is setting, its light travels to us across the earth's surface. The colors of the spectrum are gradually scattered. Red and orange are the last to be scattered before the sun finally sets.