The maximum of the color-conferring charge-transfer (CT) band in Prussian Blue (PB) varies with the electrochemically introduced cation Mz+ incorporated (as "supernumerary") for charge neutrality, and the dependence on particular properties of the Mz+ has been sought. With alkali-metal ions, the CT-maximum shifts are in the same sequence as the PB mass changes on M+ insertion; the effect on the CT ground state of the intra-lattice interaction of an M with the ferrocyanide CN- moiety (competing with cation hydration), is then implicated in shifts of the maxima, as the ferrocyanide is the donor center in the optical CT. More definitely, for M2+ and Ag+, solubility-products of the insoluble Mz+ ferrocyanides (that provide direct indicators of the intra-lattice Mz+-[Fe-II(CN)(6)](4-) interactions) show a strong correlation with the spectral shifts. The determining interaction of Mz+ with ferrocyanide within PB is enhanced in some cases by the accessibility of Mz+ oxidation states +/-1 different from the common values. PB lattice energies and the ground states of the optical US thus appear closely interlinked. The electrochemical uptake of appreciable amounts of the Mz+ within the lattices was confirmed by XPS.