Abstract: Brilliant iridescent colors occur on many biological objects. Current RGB-based graphics renderers are not sufficient to simulate such phenomena. This is because biological iridescences are caused by interference or diffraction, which requires wavelength information to describe. In this article, we propose an iridescent shading process that allows to render biological iridescences with RGB-based renderers. The key ideas are to construct spectra from colors and to use a wavelength-dependent model to describe iridescences. A novel model for iridescent illumination due to multilayer interference is developed based on analytic calculation and numerical simulation, and is simplified for practical rendering. The iridescent shading process is implemented using RenderMan embedded in Maya. Iridescent Morpho butterflies and ground beetles are rendered as examples to test the proposed techniques.