Prussian blue (PB) represents a class of metal-organic coordinated compounds with fascinating electrochromic properties. Although the electronic structure has been studied intensively, its electrochromic mechanism remains unresolved due to the lack of electronic-transition analysis. Herein, we investigate the electrochromism of Prussian blue (PB) and its derivatives by combining optical characterization and density functional theory (DFT) calculations. We unambiguously determine the optical gaps of PB-related derivatives and construct a smart window exhibiting excellent electrochromic performance and temperature control. DFT calculations demonstrate that the coloring of Prussian yellow (PY) is critically governed by two absorption bands centered at ca. 2.4 eV and 3.0 eV respectively. The former is weak and is generated by the charge-transfer transitions from the Fe(I)-t(2g) (Fe ions connected with C) band to the Fe(II)-t(2g) (Fe ions connected with N) band. The latter is strong and is induced by the electronic excitations from the Fe(I)-t(2g) band to the antibonding Fe(II)-eg band. The color change from yellow to blue is induced by the reduction of Fe(I) ions, which redshifts and enhances the former transitions but suppresses the latter transitions. In contrast, the color change from blue to transparency (Prussian white) is induced by the reduction of Fe(II) ions, which enlarges the bandgap and permits transmittance of visible light. The band-edge transition oscillator strengths are critically governed by the state weight of C-p, which provides opportunities for optical modulation via anionic doping. The insights obtained in this work are fundamental for understanding and controlling the opto-electronic properties of PB-related compounds for intelligent applications.