ISO 6271:2015 specifies a spectrophotometric method for estimating the colour of clear liquids in terms of platinum-cobalt units (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 intense than the Pt-Co stock solution the method specified in ISO 4630 applies.

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".