The colors at Norris, like in other thermal areas, are due to an interesting combination of minerals and lifeforms. Norris tends to have more milky blue features than other areas due to silica in suspension in the water (this is because the water is so hot and dissolves lots of silica). You'll also see a lot of red-orange. Some of the orange is due to cyanobacteria, but a great deal of the red-orange at Norris is due to iron oxides and arsenic compounds. There are springs at Norris that are emerald green in color; this is due to the blue of refracted light in combination with the yellow of sulfur lining the pool