Why does snow sometimes turn black?

Black snow is generally created as snowflakes fall through ash, soot or petroleum-based pollutants. Snow is also sometimes found in a variety of colors, including red, blue, orange, brown and yellow.

Red snow, also commonly referred to as watermelon snow, is caused by a species of algae that grows on snow in extremely cold regions, such as near the poles or at high elevations. Yellow, green, blue or brown snow may also be caused by algae. However, brown, orange or red snow may also be caused by the snow collecting dust or sand. Although blue snow can be caused by algae, most of the time the blue color is simply an effect of ice.