Grand Prismatic Spring in Yellowstone National Park - Temperature 147-188°F Dimensions 250x380 feet. Grand Prismatic is the largest hot spring in Yellowstone, and is considered to be the third largest in the world-New Zealand has the two largest springs. Grand Prismatic sits upon a wide, spreading mound where water flows evenly on all sides forming a series of small, stair-step terraces. The Hayden Expedition in 1871 named this spring because of its beautiful coloration, and artist Thomas Moran made water-color sketches depicting its rainbow-like colors. The sketches seemed exaggerations and geologist A.C. Peale returned in 1878 to verify the colors. The colors begin with a deep blue center followed by pale blue. Green algae forms beyond the shallow edge. Outside the scalloped rim a band of yellow fades into orange. Red then marks the outer border. Steam often shrouds the spring which reflects the brilliant colors. Grand Prismatic discharges an estimated 560 gallons per minute.