Jennifer Ouellette examines our understanding of the snowflake:
The higher the humidity, the more complex the shape, and if the humidity is especially high, they can even form into long needles or large thin plates. Scientists aren’t entirely sure why, but they suspect it has to do with the complex underlying physics of how water vapor molecules are slowly incorporated into the growing ice crystal — what Descartes termed the "ordinary order of Nature." There's still a lot of mystery in that ordinariness.
Cheryl Murphy explains why freshly fallen snow looks white:
This is because sunlight traveling to and through the airy snow is made up of all of the colors in the visible spectrum of light. This light is scattered and reflected through the many snow crystals and flakes. The hexagonal bases of snow crystals act like thousands of prisms lying on the ground, refracting and reflecting all of the colors of the visible light. In most cases, no wavelengths or colors of light are absorbed by the snow and nearly all of the light is reflected back towards our eyes which interpret all of these reflected wavelengths together as the color white.
(Photo by Jaspar Nance)