About Lightfastness
May 4, 2026
På SvenskaLightfastness, when referring to colors, means a particular color’s ability to resist change when exposed to light. All colors are affected to some degree by sunlight. Some change dramatically with exposure, while others can remain stable for a hundred years or more.
When light—especially UV radiation—hits a pigment, its chemical structure can break down. This usually results in the color fading, but it can also cause shifts in hue or even make the color disappear entirely.
A common way to assess a color’s lightfastness is to check its ASTM rating. ASTM stands for American Society for Testing and Materials. It is a standards organization that has developed methods for evaluating lightfastness, durability, and pigment stability. When you read about a particular watercolor and see the designation ASTM I, it means the color has been tested and found to be highly resistant to UV exposure.
ASTM scale for evaluating the lightfastness of artists’ colors is:
I = Excellent lightfastness
II = Very good lightfastness
III = Moderate lightfastness
IV–V = Poor to very poor lightfastness
Many online databases use a four-step scale like this:
E = Excellent
VG = Very Good
G = Good
F/P = Fair/Poor
Under real-life conditions—for example, on a wall in a normally lit room—these scales can be interpreted roughly as follows:
I or E: Minimal or no visible change over several generations (over 100 years).
II or VG: Very slight change over a long period, about 50 to 100 years.
III or G: Noticeable change within 15 to 50 years.
IV or F: Rapid change, from a few years up to about a decade.
V or P: May change within a few months to a few years.
Watercolors are particularly vulnerable to light exposure, since white paint is usually not used to lighten other colors. Instead, watercolorists dilute pigments with water to create different values. Weak (diluted) pigment layers fade significantly faster than strong, undiluted ones.
A general recommendation is to avoid colors rated lower than II or VG. From level III onward, regardless of the rating system used, the color is too unstable for use in works intended to last more than a few decades.
