Given the potentialities in terms of high bandwidth, low costs, and low power consumption, Wavelength Switched Optical Networks (WSONs) are the most promising candidate for next generation backbone networks. In WSONs the optical signal is switched at the wavelength granularity, therefore the wavelength assignment process plays a crucial role in dynamic network operation. Generalized Multi-Protocol Label Switching (GMPLS) is the standard control plane for WSONs. However, current GMPLS protocol suite does not envision a general mechanism to rank the wavelengths. The lack of wavelength preference in WSONs may cause high blocking probability, wavelength converter waste, and detrimental effects due to physical impairments.