The ONE MOST IMPORTANT thing that i have noticed brings out the vibrant red in the red bellies is low stress!!! Obtaining this environment takes a large combination of things.
1) A well varied diet.
2) Good water Quality.
3) The right tank mates.
A well varied diet of live and non-live food. I agree with many of the other posts that advocate krill my piranhas thrive on krill although it is not one of their natural prey, all my piranha go crazy almost jumping out of the water for it. They also eat worms, Large tropical flake, frozen beef heart and always live feeders. The next step to having vibrant colored fish once they have a well rounded diet is water quality.
Piranha indeed like a lower PH say 6.4 or so, adding root wood and live plants helps maintain this without adding any ph down to the water however it is most times necessary in lowering the ph. Piranha also love a ton of filtration, water movement and fresh water. These fish require a good amount of food to sustain their metabolism so you have to keep in mind that they are a large waste producing fish especially when feed live fish so keeping up on water changes is KEY!!! The more they eat the more they drink, the more waste they produce, so change their water!!!
Finally piranha are a illusive fish who seldumly live up to the hype, however that is why we all love them so much. So it is important that they get along with all their tank mates. If you have say one nasty ass serr. roaming the tank who nippes fins, eats first keeps the others hiding in the corner etc... the other fish are omiting negitave theramones which are a fishes natural predatory energy. And if suppressed by a more dominate tank mate they tend to stress the hell out and it deffinatly shows through their coloration.
I know this was a long long... post but really if you want bad ass head turning fish you must not slack on any of these matters. They are all importent in keeping these type of fish.