Sundogs have white tails often not noticed because they merge into the parhelic circle. The white comes from overlapping colours. ?Horizontal plate shaped crystals high in cirrus cloud form sundogs. The sun's rays are usually shown neatly parallel to a crystal side face. This symmetrical route is the minimum deviation condition producing the dog’s bright sunward edge. ?There are many more routes. Rotate the crystal and the ray path is no longer symmetric and - importantly - the ray's deflection is greater. These rays appear further from the sun. The rays of different colours overlap to give white.
And that's not all. Rays only pass straight through the plates when the sun is near the horizon.?At other times they reflect up and down between the large top and bottom faces before emerging.