Hair gets its color from melanin, a pigment found within the hair strands. Hair has two types of pigments: eumelanin and pheomelanin. Eumelanin has two types: brown eumelanin and black eumelanin and, like the names indicate, is largely responsible for brown and black pigments. Pheomelanin, on the other hand, is most responsible for red and yellow pigments. The ratios of these types of melanin determine the wide range of hair colors.
The more brown eumelanin and black eumelanin present in the hair, the darker the hair strands will appear, and the less brown eumelanin and black eumelanin in the hair, the lighter the hair strands will appear.
When you bleach your hair, the melanin in the hair shaft is broken down – or to put it scientifically, the bleach oxidizes the melanin, causing the melanin to lose its color. This happens when the bleach alkaline mixture changes the pH of the hair and causes the cuticle layers to lift up and open up, which allows it to pass under the cuticle and into the cortex.
Once the bleach reaches inside the cortex, it oxidizes the hair’s melanin and the oxidized molecule is colorless. The brown and black pigments are dissolved first followed by the red and yellow pigments.
The bleach will continue to lighten your hair until it is removed from your hair. If the bleaching process is not done properly, well this is where things can get ugly and that doesn't even count the few minutes of ugly crying you will go through.
If you are not careful, bleach can destroy the cuticles and cortex which can lead to dry, brittle, rough, straw-like hair. Dry, brittle hair due to over bleaching is referred to as “oxidative” or “bleaching” damage. Porosity and elasticity of the strands can also be affected if hair is over-processed. Hair that is overly porous will be hard to hold any style, which is why you may notice your hair is flat and limp and has no body. Overly porous hair will not be able to retain moisture and in extreme cases will become so spongy it will take ages to dry.