When light beams interact with particles suspended in air, some of the energy is scattered, which means the light beam changes direction, and usually color as well. The amount of light scattered is a function of the size of the particle relative to the wavelength of light falling on it. Cloud particles are large enough to scatter any color of light that falls on them. The repeated scattering of light, called multiple scattering, causes whitish light because enough light of all colors is scattered to your eye, and those colors combine to make white light.
You can demonstrate this with a glass bottle. Without the labels, the glass will look clear. Wrap the bottle in a rag, smash it, and pour the small pieces into a pile. The pile will be whitish, even though each tiny piece is clear.
The bottoms of even the whitest clouds appear gray, sometimes ominously so. Why? Because multiple scattering above the cloud base redirects the incoming sunlight out the top and the sides of the cloud, leaving very little light to emerge from the cloud base, which is dark.