The red color is caused by a kind of green algae called Chlamydomonas nivalis. Yes, I know it is red, not green, but that is because the algae also contains a red pigment. It likes to grow in freezing water, and is fairly common on old snow drifts. As the snow melts, the algae is concentrated, making the color darker.

It is sometimes called Watermelon Snow because it has the red color, and because it smells slightly like watermelon. I have even heard that it tastes slightly sweet, but that you should not eat it, as it can cause quite a bit of digestive distress.