The Reason Sunsets Are Red
When light is coming to you from directly overhead it appears white, with a small hint of
yellow. This hint of yellow is due to the removal of blue and other low wavelength colors
due to the light scattering by nitrogen and oxygen molecules in the atmosphere...
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When the sun is close to the horizon the light that reaches us has to travel through a lot
more atmosphere before it can get to us than light coming from directly overhead.  
Instead of coming through the least amount of atmosphere like the sun does at noon
when it is directly above, light from the horizon it is coming from a different angle and
travels past many more nitrogen and oxygen atoms. These atoms filter out the low
wavelength light (the blue and violet light) and let some yellow, more orange and a lot of
red light pass through.

This scattering of the blue light by the atmosphere makes the light traveling directly to us
from the sun more reddish. Looking directly into the sun to see this red light is difficult
and potentially dangerous, but if there are clouds out, or clouds near the horizon, they
will reflect some of the red light that reaches them and can create a very colorful sunset.
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