We all know that snow is a frozen ice crystals. Then why snow is white? As the frozen water is clear and transparent.
A collection of number of ice crystals are known as snow. When light falls on the layer of snow it passes from ice crystals on top. The light slightly moves from its original path then it moves onto another crystal where again it deviates. This process continues, and after bouncing from all the surfaces of the crystals light reflects back out of the snow. So all the wavelengths of light in visible spectrum (red, orange, yellow, blue, indigo, violet) combines with each other in almost equal ratio and we see the white color of snow. When all the wavelengths of visible spectrum are reflected and are combined in almost equal measure they always give a white light leaving the object appear white. When snow turns to liquid wavelength of colors passes straight without reflecting much and the whitening gradually disappears.
This can be easily understood by a simple experiment.
Create a bubble of soap you will see that it is transparent as light passes through it straight. Now make more bubbles and keep them together. They will appear white because light is reflecting after getting bounced from all the surfaces of bubbles.
Different colors of snow can be seen like sometimes it attains red color because the air may contains red dust particles. Sometimes snow appears green, orange, purple, yellow this is because of the stain given by algae.