You can paint on dye one strand at a time this way. I use a fork in one hand and the brush in the other to get the dye into the middle.
When the singles were dry I wound them into a center pull ball and then plied it taking one strand from the outside and one from the inside.
For all that work I was rewarded with a cute little 50 gram skein of extremely variegated yarn.
Not bad, since all those singles were originally destined for the trash.