This part of ISO 787 describes a general method of test for comparing the tinting strength and colour on reduction of two similar coloured pigments, the results being expressed either as "relative tinting strength" or as "equivalent colouring value". ISO 787/24 describes a general method of test for determining the relative tinting strength of coloured pigments using a photometric method. NOTE 1 When this general method is applicable to a given pigment, only a cross-reference to it should be included in the International Standard relating to that pigment, indicating any detailed modification which may be needed in view of the special properties of the product in question. Only when this general method is not applicable to a particular material should a different method for determination of relative tinting strength and colour on reduction be specified. 2 This method should not be used for those yellow pigments for which it is difficult to evaluate the tinting strength with the aid of a white pigment paste. In this case, it is common practice to use a blue pigment paste and to compare the strength and undertone of the resulting green pastes. The choice of the blue and white pigments for the blue pigment paste and its composition should be the subject of an agreement between the interested parties.