White Light is actually a mix of colours, the rainbow. You can split it into parts with a glass prism, or in good weather a sprinkler?

Light is a bit like a wave through the air and in the blue light the waves are closer together.

The sky aboves us has a small amount of dust.

Because the blue light waves are closer together they hit more dust.

This means they are more likely to bounce towards us, so we see blue when we look up at the sky.

At sunset the blue waves are more still more likely to hit dust, and so be blocked, so we see red.

Not simple enough, but I gave it a go.