My rummynoses, along with my red phantom tetras all lose their color at night. Their sense of daily rhythms is so attuned that the fading starts just before the lights go out. The lights are on a timer, so their physiology evidently is in tune with the daily light/dark cycles.
Also, I recently started using peat in my filters. The results are striking: the rummynoses red faces have deepened substantially. They are redder and the red covers more area in the head region. Peat made them happier, which makes sense for "blackwater species"