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Why are snowflakes white, if they're made out of ice - and ice is clear? No, it's not like the "cloudy" ice cubes that come out of your freezer. That's caused by tiny bubbles of air in the water that are squeezed out of solution as the water freezes. Seen under a microscope, snowflakes are clear, in fact. But instead of transmitting light, as liquid water and clear ice do, the microscopically rough surfaces of a snowflake act like tiny mirrors. Instead of passing through the snowflake, light is scattered back to an observer. Since the color of an object is determined by the wavelengths of light it reflects, and snow reflects all wavelengths, snow looks white to us.

Source: University of Illinois Physics Department; 'When snow melts, where does the white go?' by Sharon J. Huntington (Christian Science Monitor, Feb. 20, 2000).