Thank you everyone for stopping by and commenting!!
There was strong, slanting, late, golden afternoon light so that played out well in the scene - especially in the grasses. Wyoming had very pretty blue skies during my visit to Yellowstone, so many of these thermal pools were reflecting additional blue from the sky. Some of the bacteria adapted to thermal living also produce (partly) the orange layers as well as blue colours.
Akira, that graininess may be an editing artifact. It could also be sediment. I'm not entirely sure. This particular pool was very dark, so I did raise the light just a bit because it appeared as though there were some object in the water.