Have you ever seen a rainbow? Rainbows have the colors that make up light - red, orange, yellow, green, blue and purple. When light is coming from the sun, it has all these colors in them. When the light hits the air that goes around the earth, the blue lights get trapped and make the sky look blue. At night, since there's no light coming through, you can see straight out into space without that trapped blue light, which is why it looks black at night.