Scattering is caused by particles in the atmosphere. The color of the light scattered is a function of the particle size in relation to the wavelength. Cloud droplets scatter all colors in all directions. Clouds appear dark underneath - due to scattering of visible light out the sides and top of the cloud. Air molecules are much smaller than the cloud droplets.
Molecules are wavelength selective in the light they scatter.
They are more efficient at scattering light with a short wavelength violet, blue, green. This explains why the sky is blue. Dust on Mars turns the sky red with violet sunsets because these size particles scatter red light. Other examples of scattering Blueness of distant mountains. Example photo by Kari Wedel Haze particles cause the sky to look whitish.