In fruit cultivation, simple, non-destructive, and pretreatment-free measurement techniques are essential for direct field assessments. This study aimed to develop multispectral and colour imaging methods for extremely early-maturing Satsuma mandarin (Citrus unshiu Marc.) and evaluate seasonal changes in fruit quality. Over two consecutive seasons (2018 and 2019), mid-infrared (MIR) and X-ray fluorescence (XRF) spectral analyses were conducted on leaves and fruit at various growth stages, alongside reference analyses of surface colour, Brix, and titratable acidity. Consistent seasonal trends were observed across both years. XRF detected significant differences in elemental content between new and old leaves, and distinct flesh colour transitions were confirmed across growth stages. Multiple regression analysis showed that juice composition could be predicted from flesh colour, yielding leave-one-cluster-out cross-validated coefficients of determination (R²) of 0.71-0.89 (2018) and 0.75-0.96 (2019). Furthermore, total harvest sugar content per fruit was predictable from the leaf K/Ca ratio during the May fruit-set stage (R² = 0.88, cross-validated Q² = 0.71). These findings identify hue as a stable maturation indicator and demonstrate a mechanistically interpretable relationship between the leaf K/Ca ratio and harvest sugar mass, supporting classical concepts of phloem transport physiology.