Ability of a coating or of a coated timber to retain its original colour when exposed to light. Sunlight, and particularly its UV part, can alter the colour of natural wood and stains, producing yellowing of timber, and discoloration of stains. The presence of anti-oxidants and UV absorbers slow down yellowing and discoloration. Also pigmented coatings (especially white) are sensitive to yellowing over time. Light fastness can be further increased by choosing appropriate hardeners, or special coating families (such as acrylic-polyurethane coatings).