I color polyfiber batting to use as a framework for model bushes. There are model suppliers who have similar products. I have found quilt batting to be the most useful polyfiber. It is stiffer and takes the coloring better. I have tried the stuffing fiber and found it slick and harder to color.
You will need a small dish, I use cheap ones from the dollar store, quilt batting and Dye-na-flow fabric paint. It is the only paint that I have found that will leave the polyfiber workable and give an easy coating.
Cut a small pieces of batting that will fit in the mixing dish. I make several pieces at a time.
Pour a small amount of Dye-na-flow into the dish. I use black. Brown will also work. Green is best made by mixing black and yellow to give an olive drab. The Dye-na-flow greens tend to be bright.
Wear rubber gloves, the Dye-na-flow colors cuticles and finger nails very well. Swish the polyfiber around into the coloring. Squeeze it into a little ball. Pull the fibers apart.
Repeat this until the polyfiber is uniformly colored.
If too much coloring is placed in the dish it will gather in the polyfiber, as indicated at the arrow. Take the next piece and squeeze it around the first piece. Work this into the new piece.
The finished polyfiber should look like the left sample, it will be a dark gray.