Full answer
Sunlight looks white but is actually a mix of all visible colors. When sunlight enters Earth's atmosphere, it collides with tiny nitrogen and oxygen molecules. These collisions scatter light in all directions — a process called Rayleigh scattering.
Blue light has a shorter wavelength (~450 nm) than red or yellow light, and shorter wavelengths scatter far more than longer ones. Specifically, blue light scatters about 5.5 times more than red light.
Because blue light scatters across the entire sky in every direction, wherever you look upward, scattered blue light reaches your eyes. That blanket of scattered blue is what makes the sky appear blue.
At sunrise and sunset, sunlight travels through a much thicker slice of atmosphere. Most of the blue is scattered away before it reaches you, leaving the longer-wavelength reds and oranges to dominate — which is why sunsets are warm-toned.