When handpainting some BFL in my Glacier colorway today, I noticed that the blue was bleeding pink into the areas that I'd been planning to leave white. By the time I'd finished, parts of the skein were fuchsia.
So I thought I'd try rinsing the undyed sections. I definitely didn't want any pink, and I figured that rinsing couldn't make it any worse.
And it worked! Except for the bit that I absent-mindedly rinsed with hot water--that set the pink, but a little bit of overdyeing fixed that. I ended up rinsing the entire skein, which washed out the extra pink but otherwise left the colors exactly the way I wanted them. I was afraid that all the colors would run together or that everything would wash out, but instead it was the ideal outcome. Most of my attempts to fix dyeing mistakes usually make the yarn unusable, so I was very happy.