The sky looks blue because of a process called scattering. When sunlight reaches the Earth, it is made up of many colors, each color having a different wavelength. Blue light has a shorter wavelength and is scattered in all directions by the gases and tiny particles in the Earth's atmosphere.
Because blue light is scattered more than other colors, it spreads out across the sky and is what we mainly see. This is why when you look up during the day, the sky appears blue.
To put it simply, the sky isn’t actually blue itself — it’s the blue light from the sun being bounced around in the atmosphere that makes it look blue to our eyes.