Heavy detergents can strip the color molecules from the hair cuticle, exactly the opposite of what we want for our color-treated hair. Most classic shampoos contain detergents that make up around 15 percent of each bottle. Their job: to sweep away dirt and oils that leave strands squeaky clean (most commonly sodium- or ammonium lauryl sulfate and sodium- or ammonium lauryl ether sulfate). Conversely, the kinds of cleansing agents dropped into color-treated and sulfate-free formulas typically contain gentler dirt busters that won’t strip hair of color.
MORE: Are Color-Treated Shampoos Necessary? 

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