This study aimed to optimize the one-bath dyeing process for polyester/wool (80/20) fabrics using natural annatto (Bixa orellana) dye by examining the effects of temperature and dye concentration on colorimetric properties and fastness. Dyeing was carried out at 90 °C, 110 °C, and 130 °C for 30 min with dye concentrations from 1 to 30% owf, using water-soluble norbixin extracted from annatto seeds at pH 5. Color parameters (CIELab, K/S) and fastness properties (wash, light, sublimation) were assessed using standard methods. Results showed a strong interaction between temperature and dye concentration. At 90 °C, wool exhibited high dye uptake, but polyester absorption was limited, resulting in poor wash fastness despite high K/S values. At 130 °C, polyester dye penetration improved and light fastness increased, but high concentrations caused partial dye degradation and reduced color depth. The best overall results were achieved at 110 °C with 5–10% owf, balancing polyester dye uptake, wool dye retention, and dye stability while maintaining acceptable wash and light fastness. The findings highlight that the optimal dyeing conditions for polyester–wool blends with annatto differ from those for each fiber dyed separately, reflecting a compromise between polyester’s higher thermal requirement and wool’s lower heat tolerance. This work contributes to sustainable textile dyeing by demonstrating the potential of annatto to produce commercially acceptable shades on blended fabrics and by identifying light fastness as a key limitation to be addressed in future research.
