6. When you color a graph, you assign colors to the nodes of a graph so that no two adjacent nodes have the same color. For example, in Figure 2.39, if node V1 is colored red, then none of the vertices V2, V3, nor V4 could be colored red. Vertex V5, however, might be colored with red as V1 and V5 are not adjacent.

The chromatic number of graph is the minimum number of colors required to color a graph.
The chromatic number for various graphs is shown in Figure 2.40.