The lightfastness of a paint color or pigment is how permanent it is, or how unaffected by light it is.
In the United States the permanence of a color is measured by the American Standard Test Measure (ASTM), with colors rated from one to five, with I or II being the most permanent. In the United Kingdom the Blue Wool Scale is used, with colors rated from one to eight, with 7 or 8 being the most permanent. (For more detail on this, see Lightfastness Information on a Paint Tube Label.)
Watercolors, pastels, inks, and colored pencil are particularly susceptible to damage by overexposure to strong light (for instance, direct sunlight). In some museums works in these mediums are displayed under dimmed light.