Why are clouds white?

This is due to what we call light scattering(throw-in various random directions) where light strikes a particle or a collection of particles, thereby changing its direction. Clouds are made up of water or ice droplets and they are just the right size to scatter all colors of
light.When the white light from the sun hits the cloud, water, and ice droplets scatter all the wavelength of light in all directions to reach our eyes.
Note: But rain clouds tend to be grey that is because the clouds block sunlight due to their thickness.The thicker the cloud, the more light it blocks.