Striking color changes in the visible spectrum are produced when the voids of three-dimensionally ordered macroporous (3DOM) silica, alumina, and zirconia ("inverse opals") are filled with solvents, it is reported here (see Figure). The wavelength of these colors is linearly related to the refractive index of the fluid filling the pores and can be tuned by modifying the refractive index of the wall material as well as the size and spacing of the pores.