The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world, after Frying Pan Lake in New Zealand and Boiling Lake in Dominica. It is located in the Midway Geyser Basin.
Grand Prismatic Spring was noted by geologists working in the Hayden Geological Survey of 1871, and named by them for its striking coloration. Its colors match most of those seen in the rainbow dispersion of white light by an optical prism: red, orange, yellow, green, and blue.
The iconic image of Yellowstone is an expansive spring with rainbow-like colors radiating from its center, dominated by a fiery orange hue at its edges. Though these dazzlingly painted hot springs seem fit only for picture books, their colors come from very real, and very earthly, microscopic creatures.
The center of the spring is just above its underground water source, and it’s where temperatures are the highest — up to 189 degrees Fahrenheit (or 87 degrees Celsius). There, the water is too hot for most microbial growth. It is, therefore, mostly clear water. The center of the spring is blue for the same reason that the sky is blue: When sunlight hits the water’s surface, the light scatters, and blue light scatters the most, meaning that’s what reflects back to your eyes.
Moving outward from the blue center, the first ring of color is yellow, thanks to cyanobacteria called Synechococcus. The temperature of this yellow band is about 165 degrees F (74 degrees C). Carotenoids, which are also found in carrots, are orange and thus turn the normally green Synechococcus bacteria a more jaundiced shade. As you get farther from the center of the hot spring, the temperatures get lower and there is a greater diversity of microbes that can survive there. The spring’s outermost layer, at 131 degrees Fahrenheit (55 degrees Celsius), is a red-brown or burgundy color.
The scientists also reported that humans might have influenced the colors of Yellowstone’s geothermal features. In the past, the temperatures of Morning Glory Pool were significantly higher, and its color was a deep blue, they reported. As trash accumulated in the pool, somewhat clogging the vent, its temperature cooled, allowing for microbial growth and giving rise to orange-yellow microbial mats that give the pool its psychedelic appearance.
The best time to visit Grand Prismatic Spring is between May and September. Narrowing it down further, June to August will have less steam and let you see the colors even more vividly. When the temperature is cooler, the fog really builds up as the hot steam mixes with the cool air. Warm days will naturally give better views.
According to Wikipedia