Disconnected Colors in Generalized Gallai-Colorings

Gallai-colorings of complete graphs—edge colorings such that no triangle is colored with three distinct colors—occur in various contexts such as the theory of partially ordered sets (in Gallai’s original paper), information theory and the theory of perfect graphs. A basic property of GallaiQ1 colorings with at least three colors is that at least one of the color classes must span a disconnected graph. We are interested here in whether this or a similar property remains true if we consider colorings that do not contain a rainbow copy of a fixed graph F . We show that such graphs F are very close to bipartite graphs, namely, they can be made bipartite by the removal of at most one edge. We also extend Gallai’s property for two infinite families and show that it also holds when F is a path with at most six vertices. C © 2012 Wiley Periodicals, Inc. J. Graph Theory 00: 1–11, 2012 Q2