The inverse opal hydrogel heterostructure (polyacrylamide (PAAm) (left side)-polyacrylic acid (PM)/PAAm interpenetrating polymer network (IPN) (right side) is created by colloidal crystal templating. The two parts, PAAm and IPN, appear different structural colors due to the varied lattice constants and solvent response behaviors. The IPN part keeps red color and PAAm part shows different colors when the composition of the mixed solvent (ethanol and water) and crosslinking degree are changed. For example, as the ethanol content in the mixed solvents increases from 0% to 70%, the PAAm part color changes from red, yellow, green to blue when the PAAm crosslinking degree is 5 mol% or 1 mol%. Meanwhile, a large blue shift of about 200 nm can be realized covering almost the entire wavelength of visible light due to the decreased lattice spacing induced by the PAAm shrinkage. Thus, multi two-color patterns can be realized by changing the color of PAAm and the color of IPN remain red as background for contrast. Moreover, the IPN part can change from red to green by reducing the PAAm infiltration time in IPN part, which can realize the change from two-color pattern to one-color pattern at green region. (C) 2011 Elsevier Inc. All rights reserved.