Grand Prismatic Spring, in Yellowstone's Midway Geyser Basin, is the world's second largest hot spring, and is one of the most impressive thermal features anywhere. They are a number of ways to photograph it, and they fall into two categories: from the boardwalk and from the hill on the opposite side of the spring. From he boardwalk, you're mainly photographing the patterns made by the mats of cyanobacteria leading back to the spring. A wide-angle lens works best, as it is the foreground colors and patterns you'll want to capture. The 1988 fires burned off the hillside behind the geyser basin and provided a clear view of the spring. The hill is accessed from the trailhead about a mile down the road. A 28mm will allow all of Grand Prismatic Spring to be included in the composition, while a mid-range zoom will isolate colors and patterns.
From my ebook: Shooting Greater Yellowstone