Thin film interference! Perhaps the strongest and most multi-coloured example of it I have seen and the first observation I have captured myself. Now that I know what I am doing, it's actually fairly easy for me to capture it, as long as the crystal has the phenomenon.
Thin Film Interference occours when a thin layer of air is trapped inside the crystal. When reflected light hits it at just the right angle, the light coming back to the camera is refracted and split into colours much like how oil and gas makes an array of colours on the surface of water, or the rainbow effect on the surface of bubbles. Same process at work here and it takes special circumstances for a crystal to develop it. One of the harder aspects of snowflakes to capture because it requires very precise angles.
More examples to come, but this is by far the strongest example I have.

[MikeNArk]

That just adds to its beauty and to the intricate patterns of the flake. Looks like a kaleidoscope. Love that center star and colors. Great work.