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• Prussian green ECD was prepared by one-step hydrothermal method. • ECDs can show three color conversions with a single layer electrochromic material. • More active sites and shorter K+ diffusion distance improve the performance of ECDs. • The evolution of Fe in the process of discoloration has been systematically studied. • The color of the film depends on the ratio of Fe3+/Fe2+ in the PB structure. Prussian green (PG) film was directly grown on FTO glass by one-step hydrothermal process for excellent electrochromic performance. The electrochromic devices (ECDs) based on as-grown PG film can show reversible color changes between green, blue and colorless states. The existence state of iron elemental in PG film at different color states was studied by X-ray photoelectron spectroscopy. Compared with the traditional Prussian blue ECDs, PG ECDs achieve a higher optical modulation range (42.7%) and a shorter switching time (t c /t b =1.0/7.4 s). This study provides a novel method for preparing high-performance three-color ECDs by a simple hydrothermal method, and clearly shows the evolution of Fe ions during the discoloration process. [ABSTRACT FROM AUTHOR]