When all colours get reflected from the surface of an object, it appears white. In a waterfall, water drops can be thought of as suspended in air and they may form an inhomogeneous mixture of water and air. When light enters from a lighter medium (air) to a denser medium (water) some of it is reflected (by the surface) and the rest is refracted. In a waterfall, light suffer numerous reflections and refractions by the water drops because of the refractive index difference. As a result more light gets reflected. The light refracted by a layer on top would also contribute to reflection at the next layer of drops. Thus most of the light is reflected by a waterfall. This leads to whiteness. Mist, paper, water vapour, colloidal solution, clouds, talcum, snow and white paint (there are no white pigments in white paint) and sugar also appear white because of the same reason. White paint has transparent oxides of Zn, Pb, Ti suspended in transparent solution. To see waterfall white, light should not be directional (i.e.) it should be coming from all directions. If it is directional, one would see colours as in a rainbow.
