Colloidal crystals represent important templates which can be chemically modified to produce complex nanostructured materials with interesting photonic applications. The present study focuses on twopotentially useful spontaneous transformations involving a thin-film colloidal crystal array made of particles with a core-shell structure. When the colloidal crystal is exposed to moderate amounts of toluene vapors it is swollen reversibly, resulting in a marked shift of its reflectance peak. This property, combined with the system's sensitivity and the ability to repeat the process many times, makes this colloidal crystal a good candidate for the detection of toluene and similar pollutants in the atmosphere. The second transformation takes place when the colloidal crystal is exposed to higher pressures of toluene and then dried. This irreversible process yields an ordered crystal of colloidal cores embedded in a polymer matrix (an interconnected colloidal array or ICA). The simple principles driving these transformations suggest that this technique can be generalized and applied to related colloidal crystals to produce more complex and useful materials. An account of the morphology of the ICA and an explanation of the mechanism of the two transformations are presented.