Good day,
I believe the snowbow was not caused by snow but water droplets since it appears the rainbow was close to the cloud base, as snow cannot refract light the same way as droplets would to produce a rainbow. The water was most likely SUPERCOOLED, where it is still liquid below the freezing point of water.
This can be caused by two things, adding chemicals (salt, antifreeze, etc) to the water to lower its freezing point - But that is obviously not the case here - Or second, having small water droplets, held nearly spherical in shape by the surface tension of the water, and it it this surface tension that allows the water to remain liquid far below freezing (the crystal size of the ice "trying to form" is larger than the droplet itself) - and Voila - You have a rainbow (from the droplets of supercooled water) "embedded" in a shaft of snow!
The snow does not do much in the way of the rainbow, but the droplets (liquid supercooled water) mixed in with the snow and ice crystals refract light just like liquid water would above freezing, and that is what is producing the halo / rainbow.
[/b]