Hair colouring is both a chemical experiment and a personal aesthetic statement. This study investigates the molecular choreography behind oxidative and non-oxidative dyeing and examines its effects on hair structure. Using trichological microscopy, spectroscopic analyses, and published cosmetic-chemistry data, the paper reviews how hydrogen peroxide, alkalising agents, and dye precursors modify keratin, lift the cuticle, and increase porosity. Conversely, it shows how semi-permanent and direct dyes achieve colour with less structural disruption. Emerging technologies such as bond-repair additives, amino-acid buffers, and ammonia-free systems aim to preserve hair integrity while maintaining colour performance. These insights provide a biochemical foundation for balancing visual results with fibre health.