As an addendum to the above ---
Dye-fixing won't really work on a heavily-dyed item until it's been washed at least a couple of times, in my experience. Items such as these cotton indigo-dyed hakama tend to be so saturated with dye that they dry with a lot of residue on them. You have to remove some of it first through washing, then you can apply the dye-fixitive process. And hope for the best.