Believe me, I ski and walk around lots of snow, old snow, new snow, dirty snow, clean snow, and especially pink snow. I avoid yellow snow.
Clean snow seldom has any problem at all. As a result, in the winter time we melt it, but not boil it, before we consume it. Once you put a layer of muddy animal tracks across it, it might have a problem. However, we generally just scrape the top couple of inches off and then grab the clean-looking snow underneath. Sure, there could be all sorts of problems with it, but we've never found any.
In the summer, you might find pink snow, and there is a problem with it. Pink snow is actually an algae that grows on the boundary between barely melted summer snow and pine pollen. It is pink in color, and sometimes it has a faint smell of watermelons. So, sometimes it is called watermelon snow. When in melted snow, it is not lethal, but it can cause diarrhea, and that isn't good.