How well a colourant resists fading or changing when exposed to light.
Commonly rated against the ISO blue wool scale, eight reference textiles each roughly twice as fast-fading as the next. A rating of 1 fades quickly; 8 is very resistant.
Lightfastness is a property of the colourant in a particular material, not of a colour name. Two reds can differ enormously.
Many historic dyes rate 1–3 on the blue wool scale; most mineral pigments rate 7–8.
It decides how long a colour survives in a window, a gallery, a car interior or outdoors.