Iridescent colors appearing on natural objects, such as the feathers of a hummingbird, surfaces inside seashells, and shells of beetles, are beautiful and impressive. The cause of such iridescent colors is interference of light inside multilayer film structures covering the natural objects. This paper proposes a method for realistically rendering iridescent colors appearing on natural surfaces, taking into account interference of light inside multilayer structures. To render the iridescent colors, we have developed an illumination model for multilayer structures fluctuated using a hypertexture. We also have improved the hypertexture to be able to represent fluctuated parametric curved surfaces. Several examples of iridescent colors on the surfaces of a seashell and a beetle demonstrate the usefulness of the proposed method.