[Robotbeat]

Index of refraction matching (or, effectively impedence matching). Snow is white because you have a bunch of air/ice/air/ice/air/ice interfaces. The amount reflected at each interface is determined by the Fresnel Equations: https://en.wikipedia.org/wiki/Fresnel_equations

Replace the air gap with liquid water, and the slush will be much darker because the difference in index of refraction at each intereface (now liquid/ice/liquid/ice etc) will be much less.

Basically, in electrical terms, less impedance mismatch so less reflection.

Same thing works for other things besides snow.