1. each C-edge has at least two vertices with a Common color;
2. each D-edge has at least two vertices with Different colors.
In this way, "C" stands for "Common" color and "D" stands for "Different" colors.
Thus in any proper coloring, the C-edges (D-edges) represent nonpolychromatic (nonmonochromatic) subsets of vertices.
where X is the left part, C∪D is the right part, and adjacency in B(H) preserves the incidence in H.