I use natural dyes for my Easter eggs each year. Turmeric for yellow, brazilwood for red, and logwood for a deep blue-purple. The results are invariably so good that, last year, I couldn't resist the temptation to toss a skein of merino yarn in the logwood dyebath after the eggs were done, just to see what would happen.
What happened was this:
No idea how the same potful of dye that made indigo eggs could produce golden-brown yarn. It also bled like crazy because it was a spur-of-the-moment thing and I didn't mordant the yarn beforehand. I let it sit, not knowing what to do with 50g of yarn that wasn't anywhere near colourfast.
This year, when egg dyeing time came around, I remembered this skein and gave it another rinse or thirty to see whether the bleeding would ever taper off. It didn't... but thirty rinses and a soak in wool wash later, the colour had turned to a perfect chocolate brown:
It was still bleeding as much as ever, but the colour running off had changed to magenta rather than reddish.
I then decided to try mordanting it after the fact, just to see what would happen, and duly simmered it for an hour in enough water to cover the skein and 10% alum per weight of wool.
I now have this:
Yup, that really is black. I have no idea how it could come about that an originally golden brown skein could turn to chocolate brown and then actual black, all without the addition of so much as a molecule of extra dye. There must be something weird in the water hereabouts.
And speaking of water, when I rinsed the skein after mordanting, it ran off absolutely clear.
Oh, and I also did a skein of turmeric-dyed yarn this year, just for the fun of it:
Took a lot of rinsing and a soak in wool wash to stop it from smelling like an Indian restaurant after it came out of the dye, but the colour came up gorgeous.