White light is every colour at once
Sunlight looks white, but it is a blend of all the colours of the rainbow. You can see them separate when light passes through a 
prism or raindrops.
Air scatters blue the most
As sunlight travels through the atmosphere, it bumps into the tiny gas molecules that make up air. They send some of the light off in new directions. This is called scattering, and it affects short wavelengths (blue) much more strongly than long wavelengths (red).
So while red and yellow light mostly carry straight on, blue light gets bounced all over the sky. Wherever you look, some of that scattered blue is heading towards your eyes.
Wait, but violet is even shorter. Why isn’t the sky purple?
Violet does scatter even more than blue. But the sun gives out less violet light than blue, and our eyes are much less sensitive to violet. Put together, the sky looks blue to us.
Why sunsets turn orange
When the sun is low, its light takes a much longer path through the air to reach you. Most of the blue is scattered away on the journey, so the light that gets through is mostly orange and red.
Midday: short path
Sunset: long path
