Colorimetric nanosensors for monitoring food quality and shelf life provide an exciting development with obvious economic benefits. In this study, a colorimetric sensor based on silicon dioxide (SiO₂) nanoparticles and Schiff’s reagent to detect volatile organic compounds (VOCs) generated by the growth of spoilage bacteria in pasteurized whole milk stored at 7, 13, 15, and 19°C was developed. Volatile organic compounds formed from microbial growth were detected using solid-phase microextraction (SPME) and gas chromatography. Volatile organic compounds levels were correlated with microbial growth (aerobic plate counts) and color change throughout the shelf-life period. Color changes due to VOCs generated by spoilage bacteria were detected at all storage temperatures except 7°C. Nanosensor response correlated well with microbial growth in milk. Milk spoilage occurred at 32, 60, and 84 h at 19, 15, and 13°C, respectively and could be detected colorimetrically. Color change, recorded as total color difference (ΔE) for the nanosensor, correlated well with aerobic plate counts (5.0–7.0 log₁₀ cfu/mL) in milk (R² = 0.81–0.96). This colorimetric sensor could be incorporated into food packaging to predict remaining shelf life, benefitting consumers as well as manufacturers and retailers and reducing food loss.