Here's a breakdown:
* Cuticle: This is the outermost layer of the hair shaft. It's made up of overlapping scales that protect the inner layers. Direct dyes deposit color molecules on top of or just under these scales.
* Cortex: This is the middle layer of the hair shaft and contains the hair's natural pigment (melanin). Quasi-colors generally do not penetrate the cortex to a significant degree.
* Medulla: This is the innermost core of the hair shaft, but it's not always present, especially in fine hair. Quasi-colors typically do not affect the medulla.
Because quasi-colors only coat the cuticle, they are temporary. They wash out relatively easily with shampooing, as the color molecules are not chemically bonded to the hair.