Car Paint shader is based on the Renderman shader by Peter Stuart Also from the research paper A Simple Layered RGB BRDF Model by Xavier Granier and Wolfgang Heidrich. The model simulates the interaction (interference) of light with itself due to a thin substrate. To completely capture this effect, the simulation would have to be performed over the whole spectrum of light. This is obviously too much work for a shader to do, so this model avoids sampling the whole spectrum by simulating only the RGB colors and then interpolating between the results using a phong-like approach.