A subsun is situated exactly as far below the horizon as the sun is above it. A subsun is a pure reflection halo. That is why a subsun is white.
A subsun is caused by the reflection of sunlight from the upper base face of plate-shaped ice crystals which are floating in an exactly uniform way. The plate-shaped ice crystals then have the same effect like a very smooth water surface which reflects the sunlight. A subsun can also be caused by double-orientated column-shaped ice—crystals. But these crystals are very rare. As the ice crystals have to be below the observer‘s position, this halo can only be seen in freezing fog or from a plane.