What does it mean that part of my mesh is darker grey in edit mode? Boolean Problems I have been editing a mesh I created after importing an svg and using it as a boolean to cut into a plane. The plane needed lots of work to get rid of extra verts. 

As I was deleting and dissolving, I noticed that parts of the plane were darker grey. When I got rid of a few extraneous verts, it turned lighter grey, which I took to be a sign that I was doing good. 

But, I have parts that I can't get to turn lighter grey. If I knew what I was changing or what Blender was warning about, it would help me to formulate a better plan for decimating this mesh cleanly.

I have checked the normals and recalculated, flipped and set from faces etc, but I don't know what is going on.

Thanks]1
@Dimali was correct, there appears to be two faces on top of each other that share MOST of the same edges, but not these two.



I discovered this after trying to select the edge loop and those two would not be included.



Deleting those two turns the mesh a uniform grey.





Now, the edgeloops go all the way around and I can start making quads by grabbing two vertices and using J



there is probably a simpler way to "quadify" this but I haven't figured it out yet.