When we ordered the purple wool for the Thomasteppich, we decided to do a little experiment to see what the effect of the sun was on the purple colour. We hung some threads of wool outside for about two months, exposed to the weather and the full sun for most of the day (well, Dutch weather permitting ...). The effect of the exposure to the sun is enormous: the purple has bleached to a light lilac colour.
The same wool. Left the threads that were exposed to sun and weather for 2 months (therefore they are more loosely wind), right the threads in the original purple colour that was kept in a box in the dark.
This does give some thoughts on the effect it will have on the final tapestry. The Thomasteppich project will likely continue for some 10 years before it is finished (the nuns did it in 2 years time, but they embroidered every day), and embroidery is mainly done outside (though not in the full sun). However, we already see that the light green has become lighter.