Human hair is a biopolymer composed of keratin filaments, lipids, pigments, and water. Chemical treatments of hair, such as dyeing, perming, and bleaching, can impair the integrity of the hair protein structure and accelerate the loss of water and lipids, resulting in increased fragility, breakage, and rapid colour fading. This study investigates compositional changes in fermented grapeseed oil (F-GO) compared to grapeseed oil (GO) and identifies its enhanced hair permeability, highlighting its substantial potential to mitigate hair damage from bleaching and dyeing. The fluorescent labelling results verified that the ability to permeate hair was significantly improved by F-GO compared to GO and silicone oil. Fourier transform infrared spectroscopy (FT-IR) and thermal weight analyser (TGA) results illustrate that F-GO-treated hair can form additional intermolecular hydrogen bonds and might cause a conformational change in the protein structure. The in vitro assessments demonstrate that F-GO delays hair colour loss and enhances mechanical properties. The colour fixation mechanism of F-GO is attributed to its ability to act as an anti-oxygen barrier on the outer layer and a lipid barrier in the inner layer. Together, they protect against pigment loss and degradation. F-GO enhances damaged hair strength by forming hydrogen bonds with keratin residues and preserving lipids and moisture contents. Therefore, F-GO has broad application prospects for bleaching and dyeing damaged hair improvement.