Hair Dye will dye your hair whatever color you want. It can be permanent or temporary. The length of how long you want it to last will depend on the chemicals used. There are many different chemicals involved in dying your hair. The chemicals are different for every color of dye. There is a chemical called paraphenylenediamine (PPD) that people found over 150 years ago that makes brown shades when exposed to oxidizing agents. Today PPD is still one of the primary chemicals used in hair dyes.
The permanent way of using hair dye is bleaching. Bleaching your hair kills all the color cells in your hair and makes the color stay. It helps the hair dye settle in easier. There are cells in your hair that determine the color of your hair. The reactions that produce the color is carried out by alkaline pH, which has ammonia in it. Ammonia will swell up the cuticles of the hair allowing the molecules to pass through, which will dye the cells in your hair to cause the natural color to change. To actually color your hair there are compounds added to the dye. They are called couplers or coupling agents. These compounds themselves are colored but will react with the primary intermediate. Hair dyes usually combine multiple couplers to make the desired colors. There are three categories of couplers, Blue, Red, and Green.
The Chemicals in Temporary hair dye do not have the ability to get deep enough in the hair to cause permanent hair color because they don't have ammonia in them. They are absorbed on the outside. Temporary dye comes in shampoos, gels, foams, sprays and many others. With temporary hair dye it just washes out.
There are many signs that a chemical reaction is going on with hair dye. Especially in permanent dyes. While you bleach your hair it will start to feel like its fizzing and will make noises in your ears while its happening. The bleach is taking affect at this part. It makes a gross smell when applied to your hair depending on the chemicals used. The chemicals in permanent hair dye can cause many illnesses. There are articles linking hair dye to many different cancers. There are also chemicals in hair dye that people can be allergic to. These reactions can be deadly since they are constantly on you.
Hair dye is exo\thermic in the fact that it give off color light reflecting off the hair