The bulk effective dielectric function of metal-dielectric composites has a series of poles in the vicinity of which it is extremely sensitive to small changes in the properties of the components. It is possible to induce such a change in the nonlinear component of a composite, containing at least one nonlinear component, by applying an external electric field at a similar frequency. This causes a tuning of the effective dielectric function and a continuous alteration of the optical properties of the medium. We propose a general framework for calculating this effect in two component composites and also calculate it in a few three component microgeometries. It is found that the optical transmittance of a thin film of this kind can be changed from zero up to 80% using moderate electric fields.