These guys can change color REALLY quickly. If you have some tank decoration that is bright red, and put it in with him, it might help fool in into keeping its color. I recently made the mistake of putting a nice red A. pictus in just a regular aquarium....and it turned brown in about 14 days.
We added some bright red and orange plastic sponges and the angler changed from dark brown to sort of reddish brown in about a week...so at least we partially reversed the color change on this guy. Not as red as it first was, but much, much better than last week.