Pristine white snow sometimes isn't so white. Snow on the ground can be red, brown, even green. Even freshly falling snow can be bright crimson in colour, when the conditions are right. Snow falls that aren't a normal white are unusual, but not as unusual as you'd think. There's enough confirmed reports of weirdly coloured snow to average at least one such snowfall every year somewhere in the world.
Red, orange, yellow, and brown snow
Variations on red and yellow snow are by far the most common kinds of coloured snow. Red, orange, yellow, and brown snowfalls are usually caused when the dust from a nearby sandstorm gets carried over to a place where there's falling snow.
That's probably what happened during the Siberian orange, red, and yellow snow event on February 2, 2007. After all, there was a major sandstorm happening just next door in Kazakhstan. Unfortunately for the people under the snowfall, there's a lot of oil-related industries and other polluted areas between the two places. Although the snow turned out to be non-toxic, it was messy, smelly, and oily.
In spite of early reports, the yellow snowfall that was reported in Bulgaria and parts of Romania and Germany in March 2010 turned out to have nothing to do with Saharan dust. Instead, the reddish powders which gave the snow its colour were very high in heavy metals. That coloured snowfall was definitely toxic.
In Canada, Prince Edward Island snow always ends up red at least once a year. It's caused by that famous PEI red soil, the same potato-growing soil that amazes Anne of Green Gables and a lot of tourists to PEI. You can't imagine how red that soil is until you've seen it. But it's really not good for all that topsoil to end up in the air and the snow. Topsoil's a limited resource, and every red snowfall in PEI is a sign that more of it is blowing away with the wind.
A common reason for pink or red snow on the ground is the green algae Chlamydomonas nivalis. It's also sometimes called watermelon snow, because the algae actually makes the snow smell a little like a fresh watermelon. The bright red colour is caused by astaxanthin, a bright red carotenoid pigment, which gets even brighter when it's compressed by a bootprint. This kind of red snow's a common sight atop mountains and in the high Arctic, basically anywhere the temperature is cold enough for snow year-round.
These kinds of ice algae are generally non-toxic. However, you probably don't want to eat a lot of that snow, especially when you're in the middle of nowhere. Some people say that it can cause diarrhea!
Purple snow's a lot rarer than red, yellow, or orange snow. Once again, the most common reason is dust, only in this case, the dust is usually from the Sahara or Gobi Desert. To get to snowfall regions, the dust has to be very fine, so it gets lifted much higher in the atmosphere than sand can usually get.
A purple snowfall caused by Saharan dust was recorded in the Stavropol region of Southern Russia on March 9, 2010. That's right near Sochi, where the 2014 Winter Olympics are going to be held. It'd be quite a sight to watch downhill skiers racing their way through that!
During the Northern and Southern Lights, falling snow often looks green. However, that's just a trick of the light. However, polar sea ice often turns green because of ice algae such as Hormidium subtile.
It's rare, but sometimes the green algae Mesotaenium berggrenii can turn snow a gray colour. Of course, maybe it's so rare because it'd be hard to spot against older snow, which already looks kind of gray.
When volcano ash mixes with snow, the result could sometimes be black snow. There's certain to be a report of volcanic black snow in northern Japan sooner or later. After all, Hokkaido, with fifteen active volcanoes, is also home to Sapporo, the snowiest city in the world!
Sadly, black snow also falls from the sky when the region is very polluted. Norilsk, a major mining city in northern Siberia, is notorious for its black snowfalls. It's also one of the ten most polluted regions in the world.