This part of ISO 5631 specifies a method for measuring the colour of paper and board by the diffuse reflectance method with the elimination of specular gloss. It may be used to determine the colour of papers or boards that contain fluorescent whitening agents, provided the UV content of the illumination on the test piece has been adjusted to conform to that in the CIE standard illuminant D65, using a fluorescent reference standard with an assigned CIE whiteness (D65/10°) value provided by an ISO/TC 6 authorized laboratory, as described in ISO 11475. This part of ISO 5631 is not applicable to coloured papers or boards that incorporate fluorescent dyes or pigments.