Hydrogel nanoparticles that undergo volume phase transitions in response to changes in temperature and pH have been used to assemble colloidal crystal gels with environmentally tunable optical properties. When monodisperse; similar to 210-nm-diameter hydrogel particles are close-packed via centrifugation, the resultant viscous polymer pellet displays a bright iridescence in the visible region of the spectrum. This iridescence can then be modulated via temperature changes, which induce the component nanoparticles to undergo thermo-initiated volume phase transitions. More importantly, these crystals undergo a completely reversible order-disorder transition in response to larger temperature fluctuations; the crystal can be processed in its disordered (solution) state and then reformed to the iridescent crystal spontaneously upon cooling. The preparation and initial characterization of these materials are presented.