Direct dyes are applied to cellulose fibers (such as cotton, linen, viscose fibers, etc.) and protein fibers (such as silk, wool, etc.). However, the mechanisms of dyeing on these two types of fibers with completely different chemical properties are different.

Second, the dyeing time for viscose fiber is shorter than that for cotton. For the same color, the color fastness of viscose fiber is also better than that of cotton. The reason is that viscose fiber absorbs dyes more fully and deeply compared to cotton.

Thirdly, some varieties of direct dyes can be used for dyeing protein fibers, especially for dyeing dark shades of silk products. This is because the molecular structure of these dye varieties is similar to that of weakly acidic acid dyes. When they are dyed, The binding to fibers mainly occurs through ionic bonding reactions between the water-soluble sulfonic acid groups in dye molecules and the cationic amino groups dissociated from protein fibers below their isoelectric points. Its color fastness is higher than that of cellulose fibers.

The dyeing process of fibers can be described as follows: Fibers are first immersed in the dyeing solution, absorbing moisture from the solution and swelling. This swelling occurs gradually along the surface of the fibers, from the outside to the inside, and only occurs in the amorphous regions of the fibers. Dye molecules adsorb onto the fibers along with the movement of water molecules and diffuse from the outside to the inside throughout the entire amorphous region of the fibers. Over time, the adsorption deepened, and aggregation and desorption occurred. These processes took place not only between the dye and the fiber but also among the dye molecules. The adsorption and desorption that occur between dye molecules can also be called aggregation and depolymerization. The adsorption and desorption processes eventually reach a dynamic equilibrium, and this process comes to an end. Its manifestation is the process of fibers changing from "ring dyeing" to "through dyeing".