Which factor is critical in determining how hair will absorb dye?

Porosity is the critical factor in determining how hair will absorb dye, as it refers to the hair's ability to absorb moisture and products due to the condition of the hair cuticle. The cuticle is the outer layer of the hair, and when it is in good condition, the cuticle lies flat and tightly, making the hair less porous and more resistant to dye absorption. Conversely, when hair is porous, the cuticle may be damaged, lifted, or more open, allowing greater penetration of dye molecules.

Porous hair tends to absorb color more quickly and intensely than hair with lower porosity, which may require longer processing times or additional product to achieve the desired shade. Understanding the porosity of the hair helps cosmetologists tailor their dye techniques and applications accordingly, ensuring optimal results for their clients.

Factors like length, thickness, and color can influence the overall appearance of the hair and dye, but they do not inherently affect the hair's capacity to absorb dye in the same fundamental way that porosity does.