The relation between perceived image quality and naturalness was investigated by varying the colorfulness of natural images at various lightness levels. At each lightness level, subjects assessed perceived colorfulness, naturalness, and quality as a function of average saturation by means of direct category scaling. Colorfulness was found to increase monotonically with average saturation. The relation between the quality/naturalness judgments and average saturation could always be described by an inverted U-shaped function. A systematic difference was found between quality and naturalness judgments. This difference, reflecting the subjects' preference for more colorful, but, at the same time, somewhat unnatural images, was most noticeable at the original lightness level and diminished with decreasing lightness, in particular being least at the lowest lightness level investigated.