A Sky That Changes Color
Throughout the day, the sky is blue and the Sun shines a brilliant white-yellow. But as evening arrives, something remarkable happens. The Sun turns deep orange, then red, and the surrounding sky often blazes with warm shades of crimson, pink, and gold. By morning, the same drama can play out in reverse at sunrise. It feels almost as if the Sun has changed its color, but the Sun itself is doing nothing unusual. The real explanation is happening in the air all around us.
Sunlight Is a Rainbow of Colors
The first thing to remember is that sunlight, although it looks white, is actually a mixture of all the colors of the rainbow blended together. Each color travels as a different wavelength. Blue and violet have short wavelengths, while red and orange have longer wavelengths. When all the colors arrive together, our eyes see them combined as white. To understand why the sunset is red, we need to follow what happens to these separate colors as sunlight travels through the atmosphere.
A Much Longer Path Through the Air
When the Sun is high in the sky at noon, its light reaches us through a relatively thin slice of atmosphere, straight down from above. But near sunset, the Sun sits low on the horizon, and its light has to travel sideways through the atmosphere, passing through a far thicker layer of air to reach your eyes. This longer journey is the key. The same scattering process that makes the daytime sky blue is now working on sunlight that has to cross many times more air than usual.
Blue Scattered Away, Red Remaining
As sunlight passes through all that extra air, the shorter wavelengths, blue and violet, are scattered away repeatedly by tiny gas molecules. By the time the light reaches you, almost all of the blue has been scattered out of the direct beam. What is left to travel straight to your eyes is mainly the longer wavelengths, the reds and oranges. That is why the Sun itself appears red or orange at sunset, and why the clouds and sky around it glow in warm colors as they reflect this remaining red-orange light. The same scattering that paints the midday sky blue is what paints the evening sky red.
Source
This article was written using information from Wikipedia.