A method of regulating the color of Prussian blue-type metal complex nanoparticles, comprising: providing the nanoparticles each having a core-shell structure; and, adjusting the amount of the shell portion; wherein the core-shell structure has a core portion and a shell portion coating the core portion, wherein the core portion is formed from a Prussian blue-type metal complex obtained by crosslinking a metal atom M A and a metal atom M B via a cyano group CN, and wherein the shell portion is formed by mixing the so-obtained Prussian blue-type metal complex with an aqueous solution containing a metal cyano complex anion using the metal atom M C as a central metal and an aqueous solution containing a cation of the metal atom M D; wherein M A is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; M B is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; M C is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; M D is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; and wherein the shell portion has a metal composition different from that of the core portion. wherein the core-shell structure has a core portion and a shell portion coating the core portion, wherein the core portion is formed from a Prussian blue-type metal complex obtained by crosslinking a metal atom M A and a metal atom M B via a cyano group CN, and wherein the shell portion is formed by mixing the so-obtained Prussian blue-type metal complex with an aqueous solution containing a metal cyano complex anion using the metal atom M C as a central metal and an aqueous solution containing a cation of the metal atom M D; wherein M A is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; M B is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; M C is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, cobalt, nickel, platinum, and copper; M D is at least one metal atom, or two or more metal atoms, selected from the group consisting of vanadium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, lanthanum, europium, gadolinium, lutetium, barium, strontium, and calcium; and wherein the shell portion has a metal composition different from that of the core portion.