Hair is composed of proteins (keratin) embedded in a sulphur-rich matrix encased in a sheath of overlapping scales. Hair has two distinct parts: the hair bulb (root), the biologically active part of hair where new hair is formed, and the shaft, the visible yet biologically inert part of hair that is sunk in a follicle beneath the skin. As cells in the base of the follicle are pushed up, they harden and undergo pigmentation, the basis of hair color.8
Hair is made of three layers: the cuticle, cortex, and medulla. The cuticle consists of tightly packed, overlapping, colorless cells. The cortex contains varying amounts of two natural color pigments that determine a person's hair color. It also supports the physical and mechanical properties of hair, and determines its tensile strength and texture. The medulla is typically a hollow shaft inside the hair. To permanently change the color of hair, a coloring product must be able to penetrate the cuticle to deposit or remove color in the cortex.9
Natural hair color depends on the amount and distribution of pigment (melanin) in the cortex, which is hereditary. Dark pigment, called eumelanin, is responsible for brown and black color, and phaeomelanin produces blonde and red. Black hair, for example, contains densely packed melanin granules full of eumelanin pigment, while gray hair has no pigment at all.10
Hair follicles go through approximately 7-15 melanocyte seeding/replacement cycles in the average "gray-free" life span of 45 years. The average age of onset of graying for people of Caucasian descent is mid 30s, for people of Asian descent late 30s and for people of African descent mid 40s. By age 50, 50 percent of all people are likely to have 50 percent gray hair. Graying can happen gradually or suddenly, and gray hair may be more resistant to artificial color.11
Color Processes Revealed
At home or at a salon, the processes involved in coloring make the palette of possibilities limitless. Today's hair color products can remove (lift) natural hair color, add (deposit) a new color to natural color, or accomplish both processes at the same time. There are also products for adding highlights, bleaching and toning to achieve drastic color changes, and glossing or glazing for more subtle changes.
Variation in chemistry allows flexibility when it comes to levels of hair dyes.12
demi-permanent colors have little or no lightening potential. Since they use about three percent hydrogen peroxide and a non-ammonia alkalizer, the cuticle does not swell as greatly with level 3 dyes, making dye penetration less efficient.
100 percent gray coverage, even on resistant grays. Re-application is recommended every four to six weeks to avoid noticeable regrowth.