In optical networks, the information is transmitted along the optical fibers as optical signals. Wavelength Division Multiplexing (WDM) allows more efficient use of the huge capacity of optical fibers, as far as it permits the simultaneous transmission of different channels along the same fiber, each of them using a different wavelength. The connections are established by lightpaths, in which the signal is converted to the optical domain and reaches the receptor without conversion to the electrical domain. A lightpath is defined by a route and a wavelength. We assume that wavelength conversion along a lightpath is not permitted, since this technology is not yet fully available. Therefore, each lightpath should use the same wavelength from the transmitter to the receiver. The Routing and Wavelength Assignment problem consists in routing a set of lightpaths and assigning a wavelength to each of them. All connection requirements are known beforehand and one seeks to minimize the total number of wavelengths used for routing these connections, so as that two lightpaths sharing a common link use different wavelengths. In this work, we propose a new heuristic in which min-RWA is solved by a combined approach involving the computation of alternative routes for the lightpaths, followed by the solution of a Parttion Coloring Problem (PCP). Given a graph where the vertex set is partitioned in disjoint susets, PCP consists in selecting and coloring only one vertex in each subset, so as that every two adjacent colored nodes have different colors and the total number of colors used is minimum. We present and propose new heuristics for PCP and min-RWA. Computational experiments are reported comparing the new heuristics and those which already appeared in the literature.