The electro-optic behavior of isotropic colloidal crystals of spherical silica particles is investigated by the electric light-scattering method. The electro-optic responses observed at low frequencies (hertz range) are consistent with particle concentration fluctuations induced by charge transport and convection. A resonance frequency is detected, at which the decay of the electro-optic response is viscoelastic. The increase of held intensity introduces electrohydrodynamic effects similar to those observed in the liquid crystals.