In ancient Goryeo celadon excavated from the kiln sites in the GangJin and Buan areas, the effect of the chemical composition and ionic state of Fe on the colour was evaluated by Mössbauer spectroscopy and chromaticity analysis. According to chromaticity analysis, the L* value (brightness) of the glaze was shown to be affected more by TiO2 and MnO than by Fe2O3, and the body was affected more by Fe2O3 than by TiO2. The a* value was found to be affected by Fe2O3 and TiO2 in the glaze, whereas there was hardly any change in the body according to the composition. As for the b* value, changes due to the composition were shown to be smaller than those for the L* and a* values. According to the Mössbauer spectroscopy results, as the quantities of TiO2 and Fe2O3 are increased, Fe2+/Fe3+ decreases; while the changes in Fe2+/Fe3+ with MnO and P2O5 are negligible. As the quantity of Fe2+/Fe3+ increases, the a* and b* values decrease, which results in the change of colour from red–yellow to blue–green. The characteristic green colour can be attributed to increased L* (brightness) and decreased a* and b* values (blue–green shift) due to the reduced Fe ion, which is mainly determined by the TiO2 and Fe2O3 contents.