In order to explain what makes a sunset, we must first examine why the sky is blue.

On a clear cloudless day, the sky is blue because blue light scatters more than red light in air. This is due to the fact that shorter wavelengths scatter more. The reason it is blue and not violet is because there is not as much violet light generated from the sun as there is blue. So the sky does have a slight violet tint to it, but obviously is not completely violet.

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

Another factor contributing to why the sky is blue is due to our color receptors. Humans have three color receptors: green, red and blue. Therefore we recognize colors in these three receptors more so than in the other colors of the spectrum, making the sky appear blue.

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

So why do sunsets appear in different colors?

When it is a clear cloudless day, and the sun is setting, the sunset is yellow. The reason it appears yellow instead of blue is because the light has traveled a long way across the sky, and some of the blue light has been scattered away creating a yellow sky. So why does blue light scatter more than other light? In fact, blue light scatters more over long distances because smaller wavenlengths can scatter twice as much or more than that of yellow, red or orange light. This effect creates a beautiful sunset. Also, because the light from the sun is bent by the atmospheric refraction, the sun is still seen after it is below the horizon, creating an illusion.

If it is not a clear cloudless day then other factors will have an effect on the sunset such as pollution, clouds, smoke, smog, the ocean, and volcanic eruptions.

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http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html