Why does red dye fade so quickly?
Over time, dye molecules can fade due to several different mechanisms, and the most intense colors are naturally the most susceptible. Orange, pink, and even lavender shades are not all that unusual in hair that was dyed red and not properly maintained. The environment is frequently the culprit. UV radiation can react with the dye molecules inside the hair cortex, oxidizing them and leaching away the color. Other factors that can decrease color retention are pollution and heat styling, which cause similar degradation of the dye molecules.
Interestingly, the most significant contributor to color fade and loss is the rinsing away of the dye molecules by water. Since dye molecules are water soluble, water can disrupt the bond formed between the dye molecule and the hair keratin and then act as a conduit out of the hair cortex. According to scientists at International Specialty Products, red dye molecules have a greater degree of water solubility than other dyes, so they are particularly vulnerable to this effect. Surfactants from shampoos facilitate this by increasing the wetting of hair and essentially providing slippery exits for the dye through the cuticle layer. Experts at Proctor and Gamble also maintain that red dye molecules are smaller than other colors and thus have a greater tendency to escape hair readily, another reason red shades are more short-lived.
The dye process itself sets the stage for the loss of hair dye molecules. The high pH necessary for the process to work removes the protective fatty acid layer on the cuticle, rendering the hair more hydrophilic and prone to absorption of greater quantities of water than unprocessed hair. Damage to the cuticle that occurs due to the chemical process increases the porosity of the hair, which permits diffusion of dye molecules out of the cortex. Since curly hair generally has a greater initial degree of porosity in its cuticle layer, damage done from chemical dyeing can result in curlies having an even harder time maintaining red shades than their straight-haired counterparts.
