Backbone colorings for graphs

We study backbone colorings, a variation on classical vertex colorings: Given a graph G=(V,E) and a spanning subgraph H (the backbone) of G, a backbone coloring for G and H is a proper vertex coloring V ¿{ 1,2,... } in which the colors assigned to adjacent vertices in H differ by at least two. We concentrate on the cases where the backbone is either a spanning tree or a spanning path. For tree backbones of G, the number of colors needed for a backbone coloring of G can roughly differ by a multiplicative factor of at most 2 from the chromatic number ¿(G); for path backbones this factor is roughly 3 2 . In the special case of split graphs G, the difference from ¿(G) is at most an additive constant 2 or 1, for tree backbones and path backbones, respectively. The computational complexity of the problem ‘Given a graph G, a spanning tree T of G, and an integer l, is there a backbone coloring for G and T with at most l colors?’ jumps from polynomial to NP-complete between l¿=¿4 (easy for all spanning trees) and l¿=¿5 (difficult even for spanning paths)