What it looks like to me is a flipped normal. When they flip, they often get that slightly darker gray color, and in Unity they will not render at all. You can flip your normals by in the edit mode, selecting the faces that are looking odd, then at the top selecting mesh, going down to normals, and clicking flip. if all faces are selected you can recalculate them to either face inside or out at the same location. I hope this helps you.