This Part of EN 13523 specifies the procedure for determining the difference in the colour of an organic coating on a metallic substrate by visual comparison against a standard using either diffuse natural daylight or artificial daylight in a standard booth. NOTE Results may differ between natural and artificial daylight. It may occur that two colour specimens will match in daylight but not under another light source. This phenomenon is known as metamerism (see EN 13523-15). In case a metameric match is to be reported in objective terms, spectrophotometric measurements (using CIE Standard Illuminants D65 and A) are to be made, in accordance with EN 13523-15. No statement is made about either the precision or the accuracy of this procedure since the results derived are neither in numerical form nor do they provide a pass/fail evaluation in objective terms. Therefore, this procedure should only be used where the use of colour measuring instruments is not recommendable (evaluation of colour matches, inspection of metallic colours etc.). Although colour measuring instruments should be used where possible, in some cases a visual comparison can be useful (evaluation of colour matches, inspection of metallic colours etc.). The standardization of such visual comparisons, by light sources, illuminating and viewing geometry and specimen size, provides for improved uniformity of results. This practice is essential for critical colour matching and is highly recommended for colour inspections.