This part of EN 13523 specifies procedures for determining the instrumental colour difference (CIELAB) of an organic coating on a metallic substrate. Establishing a standard as well as the magnitude of an acceptable colour difference are not covered by this method. Two methods are given in this part of EN 13523: a) instrumental colour difference measurement using a tristimulus colorimeter; b) instrumental colour difference measurement using a spectrophotometer or equivalent. Care shall be taken when measuring e. g.- textured surfaces; - fluorescent coatings; - metameric coatings; - multi-coloured, pearlescent, metallic or special colour effect coatings. In order to determine whether metamerism is present, the metamerism index is determined (see EN 13523-15) and/or a visual examination (see EN 13523-22) is performed with different artificial light sources.