The purpose of this standard is to establish the method to be followed to determine the metamerism index existing between two textile materials or, in general, between two opaque and non-fluorescent solid bodies, similarly and uniformly colored in the measurement area, from the va -X, Y, Z tristimulus values obtained for two different illuminants, using any instrument that meets the necessary precision and reproducibility characteristics. The metamerism index is not used to determine the absolute color change of the same test tube when changing illuminant. It is not the purpose of this standard to quantify possible differences in color perception, caused by agents other than the materials being observed and the illuminant used, such as contrast phenomena between the samples and the environment or adaptation. chromaticity of the observer.