Lightfastness is the chemical stability of the pigment under long exposure to light. Artist quality paints are often rated according to the Blue Wool Scale (U.K) or American Society for Testing and Materials (ASTM)

Lightfastness is especially important in painting because the length of time a pigment retains its original colour saturation and value determines the life expectancy of the work of art.

Note: this should not be confused with Permanence which refers to the chemical stability of the pigment in relation to any chemical or environmental factor, including light, heat, water, acids, alkalis, or mold. For example, the pigment Ultramarine blue is extremely lightfast, but it will fade if brushed with a dilute acid.

LIGHTFASTNESS KEY:

I – Excellent lightfastness

II – Very good lightfastness

III – Fair lightfastness.