Fire stomach frog turning black?

I have a fire belly frog about two years ripened that has turned almost all black. Any thinking why? Also have one the same age that is still looking ok.
Answers:    dont verbs colour changes are natural, altho ive never seen one completely black, any haphazard of a piccy.

could be a reaction to stress or sexual maturity. just check everythings ok next to the tank and there's nothing that could be stressing it out. otherwise i wouldnt worry
Mine oscillate from bright green through all varying shades to very dark brown and they adjectives start out fire belly green
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