Hair Structure
Grab a peice of your hair. On a microscopic level that strand is a complex being with layers, that when understood can be manipulated correctly to produce amazing results. The three levels of the hair in order from outside to inside are the cuticle, the cortex, and the medulla.
The cuticle is a protective layer of dead cells on your hair. It can break and lift, so when you see split ends that's the cuticle fraying from the hair. Haircuts prevent the fraying of the cuticle at the bottom of your hair from spreading up and damaging the hair further.
The cortex contains the natural melanin (pigment) and keratin (protein) of your hair. This layer is the one we want to access when changing our hair color (darker or lighter.) We do this with the help of Developer.
The medulla is the final layer, and not everyone has it. Lighter and finer hair tends to lack the medulla, and it is usually found only in coarse/thick hair. It doesn't contribute really to what we're discussing, but it is important to know it exsists. When hair is severely damaged, odds are the medulla has been revealed and the hair will need to heal before attempting any chemical hair processes.