In Yellowstone the constantly changing weather and geothermal conditions control the light and with it your opportunities.
Grand Prismatic pool is best photographed from the air in mid day, on a sunny warm day in July or August. If the air is too cool the spring will be shrouded in fog. If you don't have a plane handy, park in the Fairy Falls trail parking lot, just to south of Grand Prismatic Pool. Take the trail about 0.6 miles. When you are just south of Prismatic Pool, climb the hill just to the south of the trail until you get a good view of the pool.