Ive kept Red Belly Pirhanas for many years and i have had groups of 5 in a massive tanks with very good conditions and i could never get them to stay vibrant in their color throughout their life. and many years ago i read something on
https://opefe.com/ that even in the wild, that it was more of a tolerated co-habitation amonst RBPs. I might be remembering it wrong, but thats kinda what i gathered after reading it. So i tried it out with a solo red belly. and it was bright red till the day it saddly died. kept solo they will be "boring" first couple of years but after that you will have a very interactive fish, that looks beautiful. but as with many piranha species patince is something you really need.