Reversible tuning of an intragap transmitting state induced by redox cycling is accomplished using a redox-active polyelectrolyte multilayer planar defect embedded in a colloidal photonic crystal (CPC). The wavelength position of the defect state can be changed by changing the oxidation state of the ferrocene moieties in the polymer backbone (see Figure). This could find applications in electrochemically tunable microcavities and CPC-based laser sources.