This is tiny snowflake, around 1 millimeter in diameter. Colors caused by thin film optical interference effect (similar to effect on surface of soap bubbles). This winter i especially hunting for rainbow snowflakes, shooting tiny hexagonal and sectored plates like this one at horizontal glass sheet, pointing macro setup at angle with small desktop tripod. After hour of trial and error, i catched some crystals with this effect. Rest of shots (quickly processed from JPEG's) available here:
This picture was made by averaging stack of 13 RAWs for better details and colors of such small object.
Version without blue color toning and vignetting:
And this is source picture after averaging 13 RAW shots, without any color/contrast changes, sharpening and noise removing (2048 x 1536 pixels):
If you want to see more snowflakes, you can browse through all snowflake pictures.
Here you'll find snowflake photo wallpapers in numerous resolutions and screen proportions, up to Ultra HD 4K.
And here is article about snowflake macro photography.