The combination of UV rays, moisture and high temperatures influence colours on textiles. This fading is an oxidation process.
Many years of research with professional aging equipment in house have led to good light fastness of these fabrics. Natural dyes are however more susceptible to change under the above influences then chemical colours. Tinctoria fabrics age gently. They will become slowly lighter when exposed to high UV, especially with moisture and heat. This proces can be directed by the user.
Direct sunlight should be avoided and lining is necessary. A second curtainrail is also an option that I would like to explain to you. In light interiors it is my advise to keep curtains closed if possible with UV values of 4 and 5.