Light from the sun is scattered by small particles in the atmosphere. But we know, the light of the sun is composed of a blend of all colors, from red, yellow, green, blue, to purple. The colors that have different frequencies. Red has a smaller frequency of yellow, yellow is smaller than the green, green is smaller than the blue, blue is smaller than purple. The greater the frequency of the light, the stronger the light that is scattered.
The sky is partly dissipated sunlight. Because most widely dissipated is high frequency color (green, blue, and purple), the sky has a mix of colors, which when combined into a bright blue.
Because many blue color dissipated, the white color of the sun is not perfect, as it should have happened if all the colors combined. The color of the sun becomes a little bit orange.
In the afternoon, the sun often turns red. At that time, the sun already slanted travel longer distances to reach our eyes, so that more light is scattered. That much is left is the light frequency
low, ie red.
In the month and on the planet that does not have the atmosphere, sunlight is not scattered, so the sky is always black, even in daylight.