Coloring polyfiber - Model Railroader Magazine

DM:

As it happens, I was just trying to dye some polyfill with black Rit dye.  I dissolved it in hot water and let the pillow stuffing soak overnight, and was able to turn it a sort of ghost gray, but not black.  I will probably end up using black spray paint after attaching it to tree armatures.

Maybe I needed hotter water. 

Edit: On the other hand, looking at Ian McM's painted polyfiber, I see that the gray I got isn't much lighter than the "highlight" color where his black paint didn't hit quite as hard...so it might work fine after all.