Adjective
lightfast (comparative more lightfast, superlative most lightfast)
Resistant to fading.
Verdigris is lightfast in oil paint, as numerous examples of fifteenth-century paintings show.
Dyes At one time acridine dyes were commercially significant, but they are now uncommon because they are not lightfast. Source: Internet
This means: * Only pastels with lightfast pigments are used. Source: Internet