The place is called Urðarháls. I could not find much information about it in the scientific literature... It is mentioned in Sigmarsson & Halldórsson (2015): they say it's an interglacial basaltic shield volcano, possibly associated with the Askja volcanic system. The fact that the volcano was eroded by glaciers probably explains the colour difference ...
What is happening here? Photoshop!
The number of visible satellites plummets towards the middle of the night when more satellites fall into the shadow of the Earth, represented by the dark area on the left of the image. (emphasis added)
According to this geological map of iceland it is the Urðarháls, an interglacial shield, i.e. a dolomite shield volcano that got scraped by glacial activity The hole in the middle is the pit crater.
You can find some cool pictures of it if you google the name with the correct icelandic letters.