Its white because there is lots of it.
A single snowflake is pretty transparent but lets say it reflects about 5% of the light(and since it reflects all light equally it reflects white light)
Now there is thousands of snowflakes and if each one reflects 5% it starts to add up quickly so lets say by the time you have 50 snowflakes stacked they would reflect over 90% of the light.
Also because the snowflakes how very intricate and diverse shape- they disperse the light in all different direction which is why the light reflected is white – and not a mirror.
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