Acrylic fibers contain anionic sulfonate groups introduced during polymerization that form strong ionic bonds with the positively charged (cationic) chromophores in basic dyes. This affinity produces very bright, high-saturation shades that are difficult to achieve with other dye classes on acrylic substrates.

When a preparation contains significant quantities of auxiliaries such as leveling agents or dispersants, the classification depends on which component imparts the essential character. If the dye remains the primary functional ingredient, the preparation typically stays within the basic dyes heading; however, if auxiliary content is dominant by weight or value, reclassification to a surface-active or auxiliary chemical heading may be warranted. Consulting a licensed customs broker is advisable for borderline formulations.