<p>Periodic color banding in the shell of the abalone <i>Haliotis discus hannai</i> may reflect environmental conditions, yet its use for age estimation remains underexplored. We tested whether hue variation on shell surfaces can serve as a proxy for seasonal growth, seawater temperature, and diet. Hue was extracted from images of eight shells from northeastern Japan, and aligned with seawater temperatures reconstructed from shell δ¹⁸O profiles and in situ records. Visually recognizable brown-to-green transitions were accompanied by concordant shifts in hue, demonstrating that banding is quantifiable. These transitions (“transition points,” TPs) occurred predominantly in May–June and recurred annually. TP-based ages broadly matched those from δ¹⁸O-based temperature–shell length alignment, with discrepancies in older shells showing multiple bands or incomplete juvenile records. Hue tracked seasonal warming and cooling phases more closely than absolute temperature levels, and appeared to mark the timing of kelp availability. Relative to isotope analysis, the TP method affords finer spatial but weaker temporal constraint near the winter minimum. Despite limitations, including occasional double bands and early-life gaps, TPs provide a cost-effective, nondestructive alternative for estimating age and inferring seasonal diet. Shell color transitions and their hue counterparts represent a practical proxy for monitoring and fisheries assessment.</p>