The sky is blue because light reaching your eye from the sky is blue. The sun sends us light that contains all the colors of the spectrum and is therefore white. Light during a sunny day passes throught just enough atmosphere to scatter most of the colors other than blue which has a rather short wavelength and is very penetrating. In the evening with the sun near the horizon, the light from the sun has to travel through much more atmosphere to reach your eye which allows blue to be filtered out or scattered. The evening sunlight passes sideways through the sky rather than nearly straight down at noon. The evening sky is often red because various colors except red which has a relatively long wave length are scattered or filtered out. A rainbow that depends on sunlight being reflected through drops of water demonstrates how various colors can be scattered in the atmosphere leaving bands of nearly pure color. Molecules in the air, especially oxygen molecules where two oxygen atoms cling together, have a similar power, although they are much smaller than raindrops.