Some dyes are excellent for fermenting, others are not. Those that are not suitable are flowers, fruity/vegetable type materials. If you want to ferment Cochineal Bugs or Madder Root, it needs to be heated every day or so to keep the mold at bay.
Things that lend themselves to fermentation are barks and tree roots. If you ferment any portioin of the Walnut or Butternut tree you will get purplish tones in you dyebaths. The longer you ferment the better. A friend of mine she's also a way excellent basket maker, Judy Dominic, fermented her walnut hulls in old glass Coke bottles for 10 years, excellent dye color. Her Osage Orange had been fermenting for 3+ years, a very old gold color was yeilded.
Osage Orange, Oaks, Maples (I got teal with my rotting maple, it was a silver maple) , Willows, Birch's, and other trees yeild their best colors only after the barks, heartwoods and root barks have fermented for 3-4 weeks or longer. If you don't introduce any contaminants mold will NOT be a problem. The tannin content is high enough to kill mold. It's the contaminants that cause mold problems, therefore I keep my dyebaths sealed while fermenting. The exception is Osage Orange and I keep it out of harms way. Sealing the dyebaths also keeps the odor down to a dull roar.
Gotta run be back in a bit with more info.