Hi! This is all to do with light. You can think of light as a wave, with peaks and dips. The distance between 2 peaks is called the wavelength. Different colours of light have different wavelengths (blue light has a shorter wavelength than red light).
Light from the sun is made up of all the different colours in the rainbow – when you add these together you get white light.
The molecules of gas in the atmosphere are just the right size to interact with the blue light, but none of the other colours. That’s why the sky is blue. The molecules in clouds are a different size and they interact with all the light from the sun so that’s why they look white.
Jack gave a great answer 🙂 Mine is below, but says the same thing really.
The sky isn’t actually blue – it’s made up of all the colours of the rainbow. The violet part of the visible spectrum has the shortest wavelength. That means it has the highest frequency and energy. As light travels in the atmosphere, most of the longer wavelengths pass straight through. Blues (higher frequencies) are absorbed more than the lower frequency colours. Light travels until it “bumps” into something – dust, water droplets or gas molecules in the atmosphere. The shorter wavelength light (mostly blue) is absorbed by the gas molecules and radiated in different directions. It scatters all over the sky and reaches your eyes – so when you look up all you see is the blue in the sky. Clouds are made of water droplets and so absorb and reflect light differently – that’s why they appear white.