<p>There is currently no standardized method for evaluating color differences between white textile samples, and existing whiteness indices are not sufficient for accurately classifying fabrics treated with fluorescent whitening agents. This study presents a reliable approach for defining industrial tolerance limits by combining instrumental color-difference formulas (∆E^*_ab, ∆ECMC, ∆E94, ∆E2000) with visual assessments. Chemically bleached cotton fabrics were treated with two fluorescent whitening agents—Texbrite BYB and Ludox White 2B—at concentrations ranging from 0.1 to 1.0% (o.w.f). Measurements included whiteness index, tint factor, and color differences. Pass/Fail thresholds were established using whiteness and color difference values, aligned with a 70% visual acceptance rate. Texbrite BYB showed wider tolerance limits (ΔE94 = 0.77, ΔWI = 4.82), but its whiteness exceeded the upper validity range of the CIE whiteness formula. Ludox White 2B had stricter limits (ΔE94 = 0.45, ΔWI = 1.57) and delivered more consistent visual results, with a 46% higher acceptance rate. A strong correlation (R² &gt; 0.75) between whiteness index differences and color difference values confirms that whiteness difference can serve as a practical indicator of perceived color variation. These results can be used as practical benchmarks for quality control in fluorescent-whitened textiles, particularly for maintaining consistent visual appearance in industrial applications.</p>