when both are ticked (and the normal colour) it means the face is aligned to the world. its not a bad thing. When one or both is/are greyed out, it means you have selected 2 or more faces with at least one aligned to something different.
Let me explain that better. Say you select either side of a wall. Unless you have changed which alignment they follow, one side will be aligned to the world, and the other to both the world and the face. this would cause the "face" check box to show grey, because not all of the faces selected are aligned to it.