The coloration mechanism of different glazes in Na2O-MgO-CaO-Al2O3-SiO2-P2O5 multi-element glaze system was revealed via investigation of the microstructure and crystallization behavior of glaze melts with different iron oxide contents. The results show that the phase separation in glaze is improved by calcium phosphate, which plays an important role in glaze color. Fe3+ ions exist at a lower iron oxide content. The chemical color of Fe3+ ions results in a dark-green glaze. The phase separation effect of glaze layer increases and the phase interface becomes larger with the increase of iron oxide content. Due to multiple scattering and consumption by massive phase interfaces, the incident light is absorbed and a black glaze appears at an excessive iron oxide content. α-Fe2O3 nanocrystals can be precipitated from silicate glaze, and glaze surface appears red color. A needle-like array structure can be formed in the growth of α-Fe2O3 nanocrystals, interfering with an incident light and resulting in silver-gray glaze. It is indicated that the interaction of chemical color and structural color induces different color rendering effects of glaze with the change of iron oxide content.