The Blue Wool Scale (BWS) indicated the lightfastness of colourants. Lightfastness is the chemical stability of a pigment or dye under long exposure to light.
A rating of between 0 and 8 is awarded. 0 denotes extremely poor lightfastness, whilst 8 denotes excellent lightfastness.
Each Colourmount board has its individual BWS rating marked on the reverse. A list of the BWS ratings for each Colourmount board is shown below: