If you are just seeing a color change, MelaFix will do nothing. It is more to cure bacterial and fungal infections. Color changes in the fish is caused from stress usually. I wouldn't treat with the melafix unless you see some sort of wound on your fish somewhere.
My pictus cats move around ALOT too when they are out, and hide out ALL day long. I never see them, except through the hole in the trunk they hide in. It's important to note that the pictus cats, as well as most other catfish, are nocturnal. It is very common that you will never see them during the day, so don't worry about that.
As to the color changing, did the color go away completley, or just get lighter? The lightening of colors usually means simple stress in the tank. What are your water parameters? How long has it been since you last did a water change? Nothing new added to the tank or anything like that? You have plenty of place for them to hide?
Answer us these questions, and we might be able to help a little more
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