A formula of the equivalent medium theory for three component metal dielectric inhomogeneous composited material is derived on the bases of the equivalent theory for two component metal dielectric inhomogeneous composite material. The new formula is used to calculate the reflectances of the sample of Au Cu Al 2O 3 and Ag Cu Al 2O 3 electrolytic coloured anodic aluminum oxide films. The calculated results are in agreement with the experimental results. It indicates that the new formula is useful at certain conditions.