Lightfastness is a property of a pigment that describes how resistant to fading it is when exposed to light. Light striking a painted surface can alter or break the chemical bonds of the pigment, causing colors to bleach or change in a process known as photodegradation.
In the United States, lightfastness is measured on the American Standard Test Measure (ASTM). The colors are rated from one to five, with I or II being most permanent. In Europe, the Blue Wool Scale is used with ratings from one to eight. Seven and eight are the most permanent.
Colored pencil lightfastness will vary greatly between manufacturers and within sets of colors by them. Oil-based pencils tend to have higher lightfastness ratings than wax-based. Water-soluble pencil lightfastness ratings vary greatly by manufacturer. Some tend to lose lightfastness when water is added due to diluting the amount of pigment in an area.