Just received my order I placed with @TheGingerDoctor from Georg Auzinger for the shirt linen and I can definitely confirm it is a very slight pale blue with white stitching detail. If anyone wants some upclose shots in different types of lighting, I would be happy to provide some!Trust me, I stared at the costume for a good half an hour, the shirt is light blue linen. There are some white threads in it, as linen is rarely a uniform color. The waistcoat has a similar weave to canvas, but it's closer to basket weave.
I don't know how she could have achieved the two very distinct different colors if she dyed it. The fibers look exactly the same, and I'm pretty sure it's wool. It's possible they could take the dye differently if they started as different colors, but neither of the colors is particularly dark and they're both pretty vibrant, not muddy. They don't look dyed at all.