HAIR COLORING SCIENCE

Hair coloring products generally fall into four categories:
temporary, semipermanent, deposit only/demi, and permanent.
All these hair color products, except for temporary color, suggest a patch test before application to determine if the client is allergic to the product.
"Hair lightening," referred to as "bleaching" or "decolorizing," is a chemical process involving the diffusion of the natural color pigment or artificial color from the hair.
This process is central to both permanent hair color and hair lighteners.
All "permanent" haircolor products and lighteners contain both a developer, or oxidizing agent, and an alkalizing ingredient as part of their ammonia or an ammonia substitute. The purpose of this is to:
raise the cuticle of the hair fibre so the tint can penetrate,
facilitate the formation of tints within the hair fibre,
bring about the lightening action of peroxide.
When the tint containing the alkalizing ingredient is combined with the developer (usually hydrogen peroxide), the peroxide becomes alkaline and diffuses through the hair fibre, entering the cortex, where the melanin is located. The lightening occurs when the alkaline peroxide breaks up the melanin and replaces it with new color.

Alternatives to Chemical Based Dyes

It has been recorded historically that ancient civilizations dyed their hair using plants. Some of the most well known are henna, indigo, Cassia obovata, senna, turmeric and amla. Others include katam, black walnut hulls, and leeks [6].
Presently, there are some companies that do sell alternate based dyes for people that are sensitive to PPD, a chemical found in most hair dyes.
There are also said to be non-toxic safer products that avoid the side-effects of chemical based dyes. The safer alternatives generally have fewer chemicals or are plant based and do also have temporary, semi-permanent and permanent options. However, these products typically do not last as long as chemical based dyes.

IF YOU WANT TO KNOW MORE GO TO:

http://en.wikipedia.org/wiki/Hair_coloring