Magnetically responsive colloidal photonic crystals have been fabricated by using highly charged superparamagnetic Fe₃O₄ colloidal nanocrystal clusters (CNCs) as the building blocks. The colloids form chainlike structures with regular interparticle spacing of a few hundred nanometers along the external field so that the system strongly diffracts light. The diffraction wavelength can be tuned across the entire visible spectrum by simply changing the strength of the magnetic field. The fast and reversible response suggests great potential of these photonic materials in applications such as displays and sensors. Further modification of the Fe₃O₄ CNCs with silica coating allows the dispersion of the particles in nonaqueous solutions. The modified particles also self-assemble into ordered structures in alcohol solvents under an external magnetic field. Embedding droplets of such a colloidal solution in a polymer matrix produces solid composite materials with magnetically responsive optical properties.