Darkening (in the male) ist mostly a matter of temper. My main filamentosus male becomes nearly black all over if he is really in spawning mood. In between he will become much lighter again.
In spec. Ampah we normally see no red at all. If the caudal contains red or reddish brown and there are marked reddish parts in the anal and dorsal it’s more likely filamentosus. The females become very light (as other licorice) in spawning mood. With spec. Ampah, there are nearly always very marked and rather broad black bands to be seen in the unpaired fins, even in the females.
I do not think that the “darkening” of Ray’s fish is a matter of the water parameters (but I would not exclude that). It’s more likely that it is a matter of temper and mood, and that means: it is a transitory phenomenon.