My discus is orange and turquoise and has always changed colors a little. In the last week, however, it will turn almost black, stay that way for a few minutes, then slowly turn back. The water temp is always in the 84 degree range.
the only thing that I can think of that was different is the tank was changed twice in one week instead of every other week (I went on vacation and my roommate thought it would be nice.)She also added a couple of catfish and danios.
When the fish is dark it shows she is not feeling well.
When it turns black it tells that the fish is sick.
Apply Acriflavine solution add salt.