The degree of lightfastness of a colour indicates the extent to which a colour-giving substance is damaged by ultraviolet light.
Ultraviolet light is a component of both daylight and artificial light. It has the ability to breakdown colouring substances; the colour 'fades'. How quickly this happens depends on the lightfastness of a colouring substance in combination with the amount of ultraviolet light. Some colours begin to fade after only a few days, others after many years or even never.
The symbols of Royal Talens should be read as follows:
º = 0 - 10 years lightfast under museum conditions
+ = 10 - 25 years lightfast under museum conditions
++ = 25 - 100 years lightfast under museum conditions
+++ = at least 100 years lightfast under museum conditions