Which type of organisms are responsible for the vibrant colors of the Grand Prismatic Pool in Yellowstone Park?

Thermophilic (heat loving) organisms.

Yellowstone's Grand Prismatic Spring (aka Rainbow Hot Spring) is characterised by its vibrant colours.

The 1871 Hayden Expedition wrote:

Nothing ever conceived by human art could equal the peculiar vividness and delicacy of color of these remarkable prismatic springs. Life becomes a privilege and a blessing after one has seen and thoroughly felt these incomparable types of nature's cunning skill.

It's all down to colourful heat-loving bacteria. Very hot water cycling from the pool's centre cools as it radiates, producing distinct temperature bands. These separate environments are homes for different bacteria; it is these thermophiles that are responsible for the myriad of colours.

https://www.smithsonianmag.com/travel/science-behind-yellowstones-rainbow-hot-spring-180950483/