Colloidal photonic crystals and materials derived from colloidal crystals can exhibit distinct structural colors that result from incomplete photonic band gaps. Through rational materials design, the colors of such photonic crystals can be tuned reversibly by external physical and chemical stimuli. Such stimuli include solvent and dye infi ltration, applied electric or magnetic fi elds, mechanical deformation, light irradiation, temperature changes, changes in pH, and specifi c molecular interactions. Reversible color changes result from alterations in lattice spacings, fi lling fractions, and refractive index of system components. This review article highlights the different systems and mechanisms for achieving tunable color based on opaline materials with closepacked or non-close-packed structural elements and inverse opal photonic crystals. Inorganic and polymeric systems, such as hydrogels, metallopolymers, and elastomers are discussed.

Poly(methyl methacrylate) (PMMA)-based colloidal photonic crystals have an incomplete photonic band gap (PBG) and typically appear iridescent in the visible range. As powders, synthetic PMMA opals are white, but when infiltrated with carbon black nanoparticles, they exhibit a well-defined color that shows little dependence on the viewing angle. The quantity of black pigment determines the lightness of the color by controlling scattering. The combined effects of internal order within each particle and random orientation among the particles in the powder are responsible for this behavior. These pigments were employed as paints, using a mixture of polyvinyl acetate as a binder and deionized water as the solvent, and were applied to wood and paper surfaces for color analysis.

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