For almost a month I left the poor little samples of plant-dyed silk baking on a sunny window to test them for lightfastness and I'm pleased to report that they kept their colours very well.
There was one surprising result, though - the silk dyed with hawthorn berries didn't fade, it didn't stay the same, instead it grew quite a bit darker and more intense in the sun. I have no idea why and no, I didn't confuse the swatches. The original is on the left, the 'suntanned' one on the right.
If you came across a similar dye reaction or can explain this mystery, I'd love to hear about it please.