With no light passing through our atmosphere (at night) the sky appears black. When there is light, the atmosphere acts like a filter allowing blue wavelengths of light through, to make the sky appear blue. The more atmosphere the light passes through the blue-er it appears. Geometrically, there is more atmosphere for the light to pass through from the horizon than if you look straight up. This means that nearer the horizon the sky appears a lighter blue than straight up because there is, in effect, a thicker filter in front of the black backdrop. This is also why, in late evening you can always see stars directly above you before the stars nearer the horizon become visible. These gradients are also affected by where the sun is in the sky of course.
I'm not 100% sure about what causes the red colourations at sunsets. I know dust that gets suspended in our lower atmosphere is a factor but I don't know if that's the only cause. The fact that the dust remains near the earth due to gravity implies that the red colourations are more likely to be found near the horizon, and only in the evening when the sun's light directly passes through this dust. You may want a more expert opinion of that bit though.
