Lightfast
Describes a colour that resists fading when exposed to light.
Also: lightfastness, permanence, colour fastness
Lightfastness is a measure of how well a pigment or dye keeps its colour in light. It is tested by exposing samples to strong light for a set time and comparing them with protected ones. Results are given on scales: artists' paints in North America are often rated from I, excellent, to V, very poor, while textiles use the Blue Wool scale from 1 to 8. Earth pigments and many modern synthetic pigments score very well. Textiles are also tested for fastness to washing and to rubbing.