Responsive core-shell microspheres of polystyrene@poly(methyl methacrylate-ethylene glycol dimethacrylate-acrylic acid) [PS@poly(MMA-EGDMA-AA] were obtained by surfactant-free emulsion polymerization and were employed for the preparation of monolithic polystyrene-based colloidal photonic crystals (CPCs) via evaporation induced-vertical deposition method. CPCs of distinct photonic stop band (PSB), which falls in the visible light region that exhibits brilliant structural colors were obtained by tuning the diameters of the core-shell microspheres. Further, as-fabricated CPCs were used for the facile, rapid visual detection of acetone by way of reversibly tuning the PSB of the CPCs with an apparent visual color change within fraction of a second. The present study demonstrates the feasibility of PS@poly(MMA-EGDMA-AA)-based CPCs as a simple, quick and reversible sensing platform for the detection of acetone.