| Slight darkening in colouration may be attributed to stress, but it
generally does not appear progressive as you have described.
Do you have a quarantine tank? I suggest you move the fish immediately
as some of the diseases that cause this symptom are incurable and
contagious.
My first guess is either an internal fungal infection (Ichthyophoniasis)
or a protozoal infestation. The later is more common in cichlids and
cyprinids and the former is known in anabantids like your gourami.
For the fungal infection fish will often waste away despite having a
good apatite and show boil like swellings that will develop on or under
the skin. You may try antibiotic foods, but Ichthyophoniasis is highly
resistant and there are no known reliable cures. Isolating the fish is
important to avoid the transference of spores to other fish. If the
fish begins swimming in uncontrolled spiraling swimming patterns the
disease has reached the nervous system and the fish should be
euthanized.
Chilodonella is an external protozoan parasite and generally causes
symptoms such as darkening of the skin and respiratory distress. This
condition is easily treated by Formalin based medications.
There are a number of internal parasites that can mimic this problem
along with some bacteria-like infections.
I suggest you isolate the fish in a quarantine tank ASAP and try
treating using a combination of QuickCure (formalin & malachite green)
and metronidazole. If the fish does not respond to this anti-parasite
treatment it may have an internal fungal or bacteria-like infection--
those that manifest these symptoms are typically very hard to treat, but
you may attempt one of a number of different broad range antibiotic
foods such as tetracycline, ampicillin, erythromycin, etc.
Good luck. I am curious to know how things turn out. | 
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