Novel opal hydrogels with water-tunable photonic bandgap (PBG) exhibiting responses to external stimuli were self-assembled from polystyrene-co-poly(N,N-dimethylacrylamide) (PS-co-PDMAA) microspheres. The polymeric microspheres with narrow size distribution were successfully prepared in water, consisting of two regions. The inner region is rich in PS which is hard and hydrophobic; the outer region is rich in PDMAA which is soft and hydrophilic. The self-assembly of the PS-co-PDMAA hydrogel microspheres is ready induced by centrifugation and resulted in highly ordered three-dimensional (3D) photonic colloidal crystals (PCCs). With an increase of the amount of water, the PBG of the opal hydrogels shifted from the visible to near-infrared region of the electromagnetic spectrum. The maximum shift of diffraction peak positions could be larger than 500 nm with narrow full width at half maximum (FWHM) in the range of 20 to 40 nm only. The change in color was visible to the naked eye. The remarkable sensitivity to water of the lattice spacing of the opal hydrogels was repeatable after centrifugation. These observations are attributed to a reproducible degree of hydration of the hydrophilic outer region of the polymeric microspheres. Furthermore, the diffraction of the opal hydrogels was particularly sensitive to the presence of thiocyanate (SCN(-)) ions. The interaction between SCN(-) ions and DMAA repeat units is argued to block hydrogen bonds between DMAA and water molecules. Our PS-co-PDMAA opal hydrogels could be a practical system for diffraction-based detections.