When traveling from east to west on the 1 towards Mývatn, you can't miss that spot. You can see the steams and colors for quite a bit.
I also marked the three hot pots in this area. Stóragjá, Grjótagjá and the Green Lagoon. Stóragjá it's not that warm anymore, so around 25°C to 27°C. Grjótagjá is closed for bathing. After an earthquake, the temperature rose up to 60°C. Now it's around 43°C to 46°C.
The Green Lagoon, well that's the thermal bath in this area. Cheaper than the Blue Lagoon.