We have developed a general method of ordering colloidal particles into three-dimensional crystals rapidly and reversibly using electric fields. The novel mechanisms involved appear to rely on a combination of induced dipole interactions and hydrodynamic flows to create close-packed, mechanically stable crystals. Because the effective interactions can be moderated using the applied field, colloids in a wide variety of size ranges can be crystallized. We show here the ordering of particles of diameter 200-450 nm and illustrate how the nucleation of these crystals can be directed through the selective application of the electric field. Since the time scale for crystallization is minutes, significantly faster than in previous methods, the crystallization method present here is expected to allow development of practical devices for photonics and other applications.