The substrate of coco fiber is perfectly fine. The reason breeders keep frogs on paper towels is to see color potentials.
If you are keeping the frog as a pet only, the fiber is great.......dirty, but fine.
When aestivation time comes around, we lower temps and place our breeders on coco fiber too. So they can dig in and get a good sleep to be ready for the next season.
They dull down in color when they dig in for camouflage reasons.......they are ambush predators and blending in is a good thing in the wild.
Our breeders are like Sumo Wrestlers.....so spoiled they don't bother to dig in unless it's time to aestivate!!