Most, if not all, of the wool we saw being used in Aubusson was dyed by Thierry Roger. His expertise was astounding. He was dyeing some wool to match a sample that had been sent to him - this is a common occurrence apparently, people send him samples of the colours they want and he dyes the wool for them rather than simply having people choose from his range. He was dyeing a batch while we were there. He would pull the wool out of the dyebath, check the colour, then add what seemed to be a few grains of dye and let it soak again. He did not measure the dye, he just used his knowledge of what he wanted and his experience with the dyes. He did this several times until he was happy with the sample.
Sometimes the artwork for a tapestry is sent to him and he dyes wool from that, ready for the weaver to use.
Of course, he does have skeins of wool that you can choose from. I couldn't resist buying one skein as a souvenir. I was a bit stingy with the amount I bought, mostly because I didn't want to fill my suitcase but also because I knew I was going to do some en plein air weaving after this and didn't know what colours I would be wanting (and I wouldn't need very much for an en plein air weaving).