Whey is an important dairy by-product whose protein content must be accurately determined for quality control and fraud detection. Conventional nitrogen-based methods (e.g., Kjeldahl) require laboratory equipment and overestimate protein in the presence of non-protein nitrogen compounds. Here, we report a rapid analytical approach based on a smart polymeric film (FRGB) prepared by diazotisation–azo coupling chemistry. The film reacts with aromatic residues of whey proteins, producing a colour change quantifiable from a smartphone image. The sensor displays a dual-range response: the blue channel provides high sensitivity at low concentrations, whereas the green channel remains linear at high concentrations, enabling direct analysis of whey protein concentrate without dilution. The method showed excellent linearity (R2 = 0.99) in whey and commercial WPC over 0–120 mg·mL−1, with a detection limit of 0.284 mg·mL−1 and recovery rate of 94 ± 1.46%. Non-protein nitrogen compounds did not interfere, allowing rapid screening and preliminary authenticity assessment of whey-derived products.
