The lightfastness rating printed on a paint tube label is an indication of the resistance a hue has to changing when exposed to light. Colors can lighten and fade, darken or turn grayer. The result: a painting that looks dramatically different to when it was created.
The system or scale used for rating the lightfastness of a paint and printed on the label depends on where it was manufactured. Two widely used systems are the ASTM and Blue Wool systems.
The American Standard Test Measure (ASTM) gives ratings from I to V. I is excellent, II very good, III fair or non-permanent in artist's paints, IV and V pigments are rated poor and very poor, and not used in artist's quality paints. (For details, read ASTM D4303-03.)
The British system (Blue Wool Standard) gives a rating from one to eight. Ratings of one to three mean a color is fugitive and you can expect it to change within 20 years. Ratings of four or five means a color's lightfastness is fair, and shouldn't change for between 20 and 100 years. A rating of six is very good and a rating of seven or eight is excellent; you'll be unlikely to live long enough to see any change.
Equivalents on the two scales:
ASTM I = Blue Woolscale 7 and 8.
ASTM II = Blue Woolscale 6.
ASTM III = Blue Woolscale 4 and 5.
ASTM IV = Blue Woolscale 2 and 3.
ASTM V = Blue Woolscale 1.
Lightfastness is something every serious artist should be aware of and decide for themselves how they want to deal with it. Know your paint manufacturer and whether their lightfastness information is to be trusted. It doesn't take much to conduct a simple lightfastness test, other than time. Decide what colors you're going to use from a position of knowledge, not ignorance, about lightfastness. While you may aspire to be listed alongside the likes of Turner, Van Gogh, and Whistler, it's surely not as an artist who used fugitive paints.
