Green faces in edit mode I pressed something and then 
The faces of my object appear green in edit mode. How can I revert that? Newly added objects appear with gray faces in edit mode.



It is not 


a matcap
a material
a texture
vertex color
object color,
You have marked your face as a freestyle face.

Either unmark it, by  RMB selecting it and pressing &#9096; CtrlF in edit mode to open the face menu. In the face menu choose clear freestyle face. You can also untick Face Marks in the mesh display options, if you only want to hide the face marks.