The sky looks blue because sunlight is scattered by gases and particles in Earth’s atmosphere. Sunlight contains all the colors of the rainbow, but blue light travels in shorter wavelengths and is scattered much more strongly than red or yellow light — a process called Rayleigh scattering, according to NASA Space Place.
That scattered blue light reaches your eyes from every direction, so the daytime sky appears blue. Near sunrise and sunset, sunlight passes through more atmosphere; most of the blue is scattered away along the path, and the remaining light looks orange or red.
Source: NASA Space Place: Why Is the Sky Blue?
Image: Clear blue sky, photo by Manishpangeni, CC BY-SA 4.0, via Wikimedia Commons.