It is feasible to realize full-color holographic display based on liquid crystal spatial light modulator by use of multi-wavelength illumination combined with time division multiplexing (TDM) or space division multiplexing (SDM). However, the modulation characteristics of liquid crystal spatial light modulator (LC-SLM) vary with the wavelength of incident laser beam. Especially, the non-linearity and the modulation-depth mismatch of phase modulation curve are the important factors influencing the reconstruction quality of phase hologram in full-color holographic display. In order to solve this problem, a novel method is proposed to correct the gray-scale of phase holograms in the RGB channels with the established gray-mapping model according to the phase modulation curves measured under different wavelengths, and compensate the effects of non-linearity and the modulation-depth mismatch of phase modulation curves. Analysis of numerical and optical reconstruction results are carried out. It is verified that the method is effective to improve the qualities of reconstructed images in the RGB channels.