Yellowstone's fantastic Grand Prismatic Spring, the largest such spring in the United States, during a moment the mists cleared sufficiently to see it, the day was quite cold and there was little wind.
The range of colors here are caused by temperature gradients, to quote Wikipedia, "The bacteria produce colors ranging from green to red; the amount of color in the microbial mats depends on the ratio of chlorophyll to carotenoids and on the temperature of the water which favors one bacterium over another" Well okay then!