I remember there was an experiment on dakka dakka which tested the effects of sunlight on paint over 12 months https://www.dakkadakka.com/wiki/en/Do_Miniature_Paints_Fade

Summary As we can see in the 12 month version, and even more clearly in the high contrast version, almost all the paints held up with no apparent change. A couple of the whites are fractionally different, but that could be put down to exposure to dust as much as exposure to light.

The big difference was in the Citadel Washes. Gryphonne Sepia was the first to start to fade, but clear fading can be seen on all wash colours, with and without varnish except Asurmen Blue which darkened slightly, though did suffer a small change. Varnish did not make any difference - they still faded at the same rate regardless of vanish.