Question

why is snow white in colour when water is colourless

Answer 1

simple
because snow is white and water has no colour so the colur of snow get mixed into water.

Answer 2

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.