Question

why snow is white while ice formed by water is colourless?

Answer 1

Light doesn't easily pass through ice but bounces around a lot and sometimes reflects back (which is why we see ice crystals sometimes having white patches in them). Snow is essentially made up of hundreds of tiny ice crystals which all partially absorb and partially scatter the light incident on them.

Answer 2

Snow appears white because light is scattered as it passes through masses of snow, and back out towards any observer. An individual snow crystal (of pure ice at least) or a cube of pure ice, doesn't exhibit this behaviour because the light is reflected off far fewer surfaces - making the crystal appear transparent.

By contrast, A mass of snow has multiple crystals of different shapes in random orientations, along with materials apart from water embedded in them. This provides a larger number of surfaces for light to be reflected off. The part of the split and scattered light that travels towards the observer, just happens to be such that it has been partially absorbed by the ice crystals, more or less uniformly across the visible light part of the electromagnetic spectrum - making it appear white.

Snow (and water) does have a bluish tinge, though. It isn't as perfectly white as it first appears. It absorbs more of the light in the redder part of the electromagnetic spectrum than the bluer part.

hope this answer is effective