ISO 6271 specifies a spectrophotometric method for estimating the colour of clear liquids in terms of Pt-Co units. It is applicable to clear liquids having a colour characteristic similar to those of the platinum-cobalt colour scale specified in Annex A. For products with colours more intensively than the Pt-Co stock solution the ISO 4630 method is recommended. The spectrophotometric method provides a more precise way of measuring Pt-Co colour than a visual sample comparison by human eyes. NOTE The term Pt-Co colour used here is preferred over the terms Hazen colour and APHA colour .