The brown in the water is sizing from the fabric. I went back to the sewing machine to finish the other two curtains, so these two soaked much longer than the second two, but it didn't seem to make much difference.
When I'd mixed the dyes, turquoise and grey, I poured them in stripes onto the fabric:
Each curtain was put into an oven bag, which protects the oven and stops them from drying out while they're in the oven:
They went into a cold oven, set to 120 degrees Celsius. It took 5 minutes to reach that temperature, I left them for 15 minutes and then switched the oven off. After another 15 minutes, I took them out, washed them in the washing machine and hung them up to dry:
I'll have to iron them before I hang them up, which may not happen today. I'm happy with how they look! It doesn't seem as though the part that was folded inside is any lighter than the outer part.