Achieving precise, localized reversible control of optical material properties is challenging. Fortunately, electrochemical reactions and proton pumping in a solid-state system provide reversible electrical control of the solid-state system's optical properties. Applying a voltage to a thin solid electrolyte layer, such as GdOx, splits water into O2 and H+ (with charge conservation ensured by electron transfer at the electrodes) at the interface between the solid electrolyte and an electrode. The voltage drives the protons into the solid electrolyte, changing the solid electrolyte's refractive index. Reversing the polarity of the applied voltage drives the protons out of the solid electrolyte, reversing the refractive index change. This reversible electrical control can be used to implement interference color modulation, transmission modulation, and switchable plasmonics. Because the solid electrolyte can be less than 10 nanometers thick, this electrochemical control enables highly localized control of optical properties active plasmonic devices and reconfigurable metamaterials.