To understand how color works, you have to start with the structure of hair. Each strand is made up of three distinct layers: the outer layer, or cuticle of the hair, the body of the hair shaft, called the cortex, and the innermost layer, called the medulla. These layers are made from the protein keratin, which is made up of 18 amino acids. The cuticle layer protects the hair and determines how shiny or dull it will look, while the cortex is the main body of the hair, holding in moisture to help it feel strong and resilient.
Beyond hair structure, it’s important to look at how hair behaves from person to person. There are many different hair types, and artificial color results can vary depending on the hair.
Texture is the diameter of a single strand of hair, and varies by genetics as well as changes with age. In addition to diameter, texture refers to the shape of a hair’s cross-section: curly hair has a flat cross-section, while straight hair has a round section.
How Texture Affects Hair Color
Very coarse hair has a slower, harder time absorbing color while fine hair receives color easily.
Porosity refers to hair’s ability to absorb liquid, and plays a significant role in the effect of permanent color. Increased porosity in hair is often the result of damage from heat styling, sun exposure, bleach, and other drying factors. It’s important to know one’s porosity level to not only care for one’s hair, but also prepare it for color.
To determine this, simply take a strand of hair and slide fingers from the end upward. If it feels rough, especially toward the ends, your hair is porous. You can help repair overly porous hair by using nourishing oils and deep conditioning treatments containing keratin to help repair any damage, as well as limiting exposure to harsh environments.
How Porosity Affects Hair Color
A porous strand of hair has a lifted or frayed cuticle, which means the hair shaft is not protected, making it easier for permanent hair color to penetrate the hair shaft and deposit a lot of color. This can further weaken already fragile strands and can affect the color result.
Hair that is not porous is called resistant, and often repels and requires more time to process. When coloring hair, it’s important to factor in the hair’s porosity. For highly porous hair, particularly the ends, choosing a shorter processing time will help provide rich, balanced color. For resistant hair, a longer processing time will ensure that color is properly absorbed, and that any grays are adequately covered.
Hygroscopicity is similar to porosity in that it measures the ability to absorb water vapor. It's what makes hair frizz in humidity.
Elasticity is the hair's ability to stretch and return to its original shape.