GB/T 7771-2008 "Determination of Special Metamerism Index: Change in Illuminant" specifies the method for determining the special metamerism index under changing illuminant conditions, which is applicable for assessing the degree of metamerism of object colors. By measuring the color difference between the sample and the reference standard under different standard illuminants, the special metamerism index is calculated. The detection requires the use of color measurement instruments and standard illuminant devices that meet the requirements, providing technical support for the color matching quality of products such as dyes and coatings.