Strong polyelectrolyte photonic hydrogels have emerged as a promising platform for highly sensitive colorimetric sensing of heavy metal ions. These hydrogels possess both the polymeric network structure and the polyelectrolyte properties, which enable them to undergo significant volumetric changes in response to the presence of heavy metal ions. This volumetric change is associated with a color change due to the structural rearrangement of the colloidal arrays within the hydrogel. The sensitivity of these photonic hydrogels is attributed to their ability to specifically interact with heavy metal ions through complexation or coordination interactions. This interaction leads to the aggregation or complexation of metal ions with polymeric chains or network junctions, resulting in a macroscopic response that can be observed visually. The aggregation or complexation-induced volumetric change alters the interparticle spacing within the colloidal arrays, leading to a shift in the diffraction wavelength and a corresponding change in color.