The nature of the origin of the color in anodized aluminum colored by ac electrolysis in electrolytes containing nickel, copper, silver, gold, and molybdenum salts is investigated. The measured specular reflectance spectra in the region 350–750 nm can be adequately explained by assuming that small metallic particles are incorporated in the anodic film as a colloid. By calculating the optical constants of such a system, specular reflectance spectra can be deduced. A detailed comparison of the calculated and experimental reflectance spectra yields good agreement. It is concluded that metals which do not possess interband transitions or plasma resonance absorptions in or near the visible region of the spectrum will produce brown or bronze colored anodic films when incorporated as a colloid.