ISO 4630 specifies a method for estimating the colour of optical clear, yellow/brownish coloured liquid products by means of the Gardner colour scale using colour-measuring instruments. The test uses the Gardner colour scale described in Annex A. The method is applicable to drying oils, varnishes and solutions of fatty acids, polymerized fatty acids, resins, tall oil, tall oil fatty acids, rosin and related products. The results might be invalid if other products are tested. The method described provides a more precise way of measuring Gardner colour than a visual sample comparison using human eyes. It is applicable to products having colours from Gardner 1 to Gardner 18. The Gardner scale is not applicable to products with colours darker than 18. For products with colours lighter than Gardner 1 the ISO 6271 method is recommended.