Don Komarechka captures astonishing photographs of snowflakes. His book Sky Crystals is a survey of snowflake science, a monograph of his macrophotography masterpieces, and a tutorial on the techniques. At Petapixel, Komarechka explains the surprising pop of color sometimes seen through the lens when he's shooting a snowflake:
As a snowflake grows it often creates a cavity or bubble inside of it where the inner side of the crystal grows slower than the top and bottom edge. This forces the layers of ice on either side of the bubble to be incredibly thin, so much so that light will interfere with itself.
Some light will reflect off the surface of the snowflake, but some will also enter the ice (slowing down due to the density of ice compared to air) and reflect off the inner ice/air boundary back towards the camera. If the ice is thin enough, the distance between the two rays of light is close enough to force them to interfere with each-other now that they are out of sync. Some wavelengths get amplified and others get reduced, resulting in a distinctive color emerging based on the thickness of the ice.
"How I Capture Vibrant Colors Inside Snowflakes" (PetaPixel)