Intelligent packaging is gaining increasing attention as a sustainable approach to enhance food safety, monitor food quality, and minimize waste throughout the supply chain. An agarose-based biopolymeric hydrogel was developed and optimized as a platform for integrating ferric reagent mixtures as colorimetric indicators. The interaction between the hydrogel-embedded reagent and phenolic compounds associated with food quality was systematically examined under controlled conditions. Optimization of the hydrogel's composition provided suitable porosity, transparency, and stability, enabling uniform distribution of the indicator and consistent, reproducible color responses. Visual changes were documented using a smartphone camera, followed by quantitative evaluation in the RGB color space. Color intensity values were converted into greyscale and correlated with phenolic concentrations. The system was tested up to 1 µM phenolic concentration and exhibited a clear concentration-dependent response with good reproducibility. Results from smartphone-based digital image analysis showed strong agreement with spectrophotometric measurements, confirming the accuracy, sensitivity, and reliability of the approach. The hydrogel sensor responded rapidly, required minimal sample preparation, and maintained stability during storage and handling. Its straightforward design and environmentally benign composition underline the practical feasibility of incorporating such systems into real-world food monitoring practices. Overall, biopolymeric hydrogel–colorimetric platforms represent a sustainable and promising alternative to petroleum-based packaging materials. Their integration into intelligent packaging systems has significant potential to enhance food safety, improve consumer confidence, minimize waste, and contribute to circular economy goals by combining eco-friendly materials with real-time freshness and quality monitoring.