Why is the sky blue in the day and orange/red at sunset/sunrise?
The light we receive from the sun has many wavelengths, the most intense of which are in the yellow-orange range. So why is the sky blue?
As the sun is seen as higher in the sky, the light radiation approaches the viewer in direct straight lines. As the light enters the atmosphere it encounters particles; these can be dust particles or molecules of gas, etc... The light is "scattered"; this is called Rayleigh scattering. The shorter wavelengths (blue and violet) are preferentially scattered so the sky is perceived to be blue.
So why is the sky at night often reddish? First, the light near sunset (and sunrise) approaches the viewer from the "side" -- it must pass through a lot more atmosphere than if it were coming from high above. Now the longer wavelengths get through while the shorter wavelengths are scattered for a perception that the sky on the horizon is reddish.
Another phenomenon also occurs which is called diffuse sky radiation or Mie radiation. This is the light that reaches the ground after having been scattered. This light does not show a color preference; it is the reason that an overcast sky is gray.
Suspended dust particles in the upper atmosphere can also create a vivid red effect at sunrise or sunset. Volcanic eruptions occasionally send particles into the upper atmosphere (as can large storms) that can create additional scattering producing very red skies along the horizon.