A novel aqueous solution of hydrogel nanoparticles with interpenetrating polymer networks of poly(isopropylacrylamide) and poly(acrylic acid) has been synthesized. This hydrogel can form a physically bonded nanoparticle network above a certain gelation temperature. The system exhibits a very rich phase behavior, including a colloidal crystalline phase, in which the system displays iridescent colors (for examples, see Figure).