A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.

Why IS the sky blue? Why is the colour of the sky blue? Two answers - you decide which is the true answer. 1 The light from the sun is a mixture of red, orange, yellow, green, blue and violet. You can see these colors when you look at a rainbow in the sky. But when the different colours are mixed together they appear to be white (not really white). But the clouds reflect the blue wavelength of the sun's light and the blue light bounces around the air, clouds and other gases and particles that are in the atmosphere. The other wavelengths pass through the atmosphere. 2 The sky is blue (sad) because we are putting too much and too many green house gases in our atmosphere and the sky is not happy anymore. There are more than 70 green house gases in our atmoshere, including carbon dioxide CO2, methane CH4 and nitrous oxide N2O.

Because it is blue,so it is blue.xixi :)

Because it isn't green.