Although I also can't talk with my fish...yet, anyway
...I can say with some degree of certainty that a darker substrate will be beneficial to any SA characin with very few exceptions. I have personal experience.
I have a group of wild-caught Hyphessobrycon metae in my 90g flooded Amazon forest setup. These fish (and all the other fish and plants in this tank) were previously in my 70g (photos of that tank are still under my "Aquariums" for reference as "Former 70g") which had a natural buff coloured gravel. Lots of plants and wood as the photos indicate. The H. metae were rarely out swimming, spending most of their time among the plants at the sides and back. In July I moved my 90g and reset it as the new home for these fish and plants, and used the darker gravel in the 90g. Within a couple days of being moved, the H. metae were out swimming as a group, and now I rarely see them among the plants which are still there for them. The fish also darkened a bit, due to the darker substrate. I've had other characins and corydoras who have displayed much the same behaviour and colouration change over darker gravel.
The reason is obvious; these fish occur in dimly-lit forest streams that are overhung with vegetation. The direct sun rarely penetrates the water. Cardinals in particular are known to inhabit very dark waters; photographs taken by underwater photographers show these fish appearing as the blue neon line in the murky water; the fish body cannot even be seen, only the bright neon line moving around like sticks of blue/green light. Nature has obviously designed these fish for such an environment. And while we do not want murky aquaria in which nothing is visible, we can provide less bright lighting and dark substrates, plus lots of plants, to provide something closer to their natural environment. And neons require much the same.
Quickly on the light, provide the minimum necessary for plant growth, and have floating plants. Plants do not require the high light level frequently advocated, as my 20 years of successful planted aquaria along with many other aquarists' experiences clearly proves. And by the way, simplicity works better--lots of plants stabilizes the water and provides a suitable habitat for tetras. And on the water, cardinals are much less tolerant of differing water parameters than are neons. The latter can adapt fairly well to slightly basic (alkaline) water, but cardinals for long-term health must have soft acidic water. They occur in streams with a pH of 4-5 and no hardness whatsoever; in aquaria they tend to live best in a pH in the low 6's maximum, but still require very soft water. Harder water causes calcium blockage of the kidney tubes. Most find cardinals live 2-3 years, but Dr. Jacques Gery had cardinals that lived beyond ten years in his aquaria, solely because they had the necessary water parameters.
One comment on temperature; cardinals prefer warmer aquaria, anything around the usual community temperature of 78-79F or higher if the other fish require it (when kept with discus or Mikrogeophagus ramirezi for instance). Neons on the other hand come from streams that are slightly cooler, low to mid-70's F suits them better. A couple degrees may not seem like much to us, but to a fish forced to live in this temperature constantly and unable to regulate its body heat, a couple degrees can mean the difference between a long stress-free life or a shorter stress-filled one. Temperature requirement is only one of the water parameters that should be considered when selecting fish for a "community" aquarium. Fish having similar requirements respecting water and environment (wood, rock, plants...) will be more relaxed because they will be "in their element" as we say, and that means less stress which leads to improved health and life.