In June 2011 I started to test the properties of Arches Torchon Watercolour paper and Winsor&Newton paint. I painted the colours Payne's grey, Sap Green, Sepia, Crimson, Ultramarine, Lemon Yellow, Winsor Blue, Winsor Red and Winsor Orange) in strokes, cut the paper in three pieces, put one in a binder, varnished one (with Schminck aquarelle varnish) and did nothing to the third.
Then I stuck the varnished and untreated parts to a window on the South, where the sun is practically always shining (and it is a bright hot sun: I'm in the South of France).
Last year (2014) the Alazarin Crimson was definitely a bit paler and also the Ultramarine seemed to be a bit lighter, especially on the un-varnished part.
This year, if I squint, it looks as if both parts (left and right) that have been in the sun (for 4 years now), are slightly lighter than the part (middle) that hasn't . But it could just be the photo...
We'll probably see next year, because they're back on the window!
The paper (Arches 300grs torchon or rough grain) hasn't changed colour after 4 years by the way.
I'll soon start with part two of the research: other brands, other paper...
More next year!!