To study the coloring mechanism of the light greenish-blue glaze used in celadon shards from the Longquan kiln from the Southern Song and Yuan Dynasties, celadon samples were analyzed using colorimetry, XRF, XRD, XPS, and SEM/EDS. The results showed that the a* and b* values of the light greenish-blue celadon glaze from the Southern Song and Yuan Dynasties were smaller than those of the celadon glazes from the Ming Dynasty, while the value of L* value from the Southern Song and Yuan Dynasties was larger than that from the Ming Dynasty. The opaque and jade-like appearance of the light greenish-blue celadon was mainly caused by the effect of light on small and dense air bubbles and the large number of quartz crystals in the glaze. Fe2O3 dominated the color of Longquan celadon, and the ratio of Fe2+ to Fe3+ in the light greenish-blue celadon glaze from the Southern Song and Yuan Dynasties was higher than that of the celadon from the Ming Dynasty. This was the major reason why the light greenish-blue celadon from the Southern Song and Yuan Dynasties was bluish and why celadon from the Ming Dynasty was yellowish-green. However, the phase-separated structural color had little influence on the color of the light greenish-blue glaze in the Longquan kiln from the two dynasties. The color of the celadon from the Southern Song and Yuan Dynasties was mainly chemical in nature.