The pigments used in Derwent products are tested for lightfastness and graded from 1-8 (8 being considered extremely lightfast). We use the Blue Wool standards - a test developed for the textile industry.
Blue wool ref 1 will fade very quickly, whereas blue wool ref 8 is considered extremely lightfast. Any result above blue wool 6 is considered to be GOOD and will last in excess of 100 years in gallery conditions.
Next to the colour name on the colour chart there is the corresponding LF rating.
See Colour Chart