The crazy colours you see in this snowflake are caused by bubbles in the ice, which in turn create extremely thin layers of ice. These ice layers are thin enough to make light interact with itself in some interesting ways:
Some of the light will reflect off the surface of the ice, but some light will pass through. Of the light that enters the ice, some of that light will reflect back off the inner boundary of the ice, and rejoin the beam of light that had originally reflected off the first surface.
The light that entered the ice, however, began traveling slower through the denser material. It speeds up again when it enters the air, but its position is off-set from the wavelengths that reflected off the surface. This is called a "phase shift", and it will make different wavelengths of light interact - amplifying some colours and diminishing others. This creates the colours that you see!
Why are there multiple colours? The colour you see is directly related to how thick the layer of ice is. As the ice thickness changes, the phase shift of the light will also change and generate different colours.
Bonus fact: It's no coincidence that the area where the two snowflakes overlap is brighter. In this area, there are more layers of ice capable of reflecting light back to the camera.
Photographic techniques: This image was comprised of 30 separate hand-held frames, required to get the entire crystal in focus. These images are combined through a lengthy editing process in Photoshop with finishing touches in Lightroom. The entire editing process takes about 4 hours to complete for each snowflake.
The image is made at a fairly extreme magnification, about six times closer than the average macro lens can get. The depth of field at this scale is razor-thin, a small fraction of a millimeter. Hundreds of images are taken of any given snowflake to ensure that I get everything in focus in at least one frame, then the necessary images are chosen for focus-stacking afterwards. All of my snowflakes are lit with a ring flash and photographed on an old black mitten. :)
Intrigued by this? Much more in my new book, which will be shipping next week! I've just heard from the print facility that I'm scheduled to receive the new print run of books on December 3rd, and I'll be shipping out books to everyone immediately thereafter.
Grab your copy here! http://skycrystals.ca/product/pre-order-sky-crystals-unraveling-the-mysteries-of-snowflakes-book/