Dye precursors that combine with hydrogen peroxide to form larger, permanent dye molecules are called _____.

• A. Azo dyes
• B. Leuco dyes
• C. Nitro compounds
• D. Aniline derivatives

Answer: (D)

Leuco dyes are dye precursors that exist in a colorless form and react with hydrogen peroxide during the oxidative dyeing process to form larger, permanent dye molecules. In hair coloring, the leuco base is oxidized by the developer (peroxide) to create the pigment that deposits in the hair and remains as long as the molecules stay in the hair shaft. Other terms describe different dye classes or functional groups, but the specific colorless-to-colored transition driven by peroxide is defined as the leuco dye form.