Abstract: A method of producing Prussian blue metal complex nanoparticles and Prussian blue metal complex nanoparticles obtained by the method, a dispersion of the nanoparticles, a method of regulating the color of the nanoparticles, and an electrode and a transmitted light-regulator each using the nanoparticles. Prussian blue metal complex nanoparticles are produced by: mixing an aqueous solution containing a metal cyano complex anion having metal atom MA as a central metal and an aqueous solution containing a cation of metal atom MB; thereby precipitating the crystal of a Prussian blue metal complex having the metal atom MA and the metal atom MB; and then mixing the Prussian blue metal complex with an aqueous solution containing a metal cyano complex anion having the metal atom MC as a central metal and/or an aqueous solution containing a cation of the metal atom MD.