I can’t comment on the discoloration but the fly-dusting schedule may not help. Just use Repashey Calcium Plus every feed.
Also, how are checking humidity? 90-99% seems high and may indicate a faulty hygrometer (extremely likely!).

 

I use exo terra combo thermometer/hygrometer. I replace them about 2-4 times a year it feels like lol. Thank you!!

 

Pictures of the full tank will be helpful

 

Bioactive vivarium 18x18x18. Misted every other day.

 

Tank looks exceptionally dry and is way smaller than, I think, any of us would recommend for Dendrobates tinctorius.

 

18 inch x 18 inch x 18 inch was suggested to me by every source. Hygrometer also reads 98 % humidity.

 

Unfortunately, 18x18x18 get recommended alot for darts, which just isn't good advice. Additionally, darts in general, but especially tincs, are suggested to be more terrestrial, which again, just isn't true. Any dart species will use every square inch, height included, in a well designed vivarium.

Hydrometers aren't really reliable, and often times give readings that are much higher than what the actual humidity is. This leads to less misting than required and dry tanks.