Here’s probably the most colourful snowflake I have ever photographed. It’s tiny, but nearly the entire body of the crystal is generating this vibrant rainbow effect. View large!
The colours you see here are created by a bubble of air in between two different layers of ice. As the thickness of the bubble changes, so to does the colour. This is due to optical interference, much the same phenomenon you’d see in a soap bubble or oil slick. If the angle of light is perfect, the patterns can be quite bright. This snowflake measures much less than 1mm across.
Not all snowflakes can generate these colours, but typically if I find one “colourful” snowflake, I’ll find another few from the same storm system.