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A 
-coloring of a Graph 
 is an assignment of one of 
 possible colors to each vertex of
 such that no two adjacent vertices receive the same color.
See also Coloring, Edge-Coloring
References
Saaty, T. L. and Kainen, P. C.  The Four-Color Problem: Assaults and Conquest.  New York: Dover, p. 13, 1986.