By means of LMTO-ASA method and the supercell approach, we have calculated the electronic structures of the three stable phases in Ti-N system, i.e., α-Ti, ε-Ti2N and δ-TiN. Then, we chose these stable phases as the samples of the Ti-N system and used the statistical supercell method to calculate the electronic structures of TiNx system. Furthermore, the joint density of states for TiNx system have been calculated and the results have been utilized to investigate the optical properties of TiNx system. In comparison with the joint density of states for gold, we found that the TiNx films present golden colour in the range of x from 0.4 to 0.6.