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.