They look like rbps, regarding the color from what i've been told is also a balance of what you feed them. From what i have read and been told that if you feed them shrimp it brings out there color, also the freeze dried shrimp you can buy also brings out there color, my red bellies atm all have really red bellies as i have been feeding them according to what Jp recommended which was Fish Fillet, Shrimps & Freeze Dried Shrimps. I also give them a cube of Blood Worms every 2nd day. Out of everything i have fed them they love the real shrimps the best.
Regarding the picture being a Pacu or rbps from what i have experienced is that the rbps have a black white black tail where the pacus have white and black tip but i am no expert and i am not saying that you are wrong Dio but i also noticed that pacus have a larger anal fins which comes out more than the rbps, but nonetheless this is only a observation i have made watching the rbpacu grow from 2.5" to 4" and watching my current rbps that are around 2.5". Also if the picture were really clear could see better.
P.s Dio is right if the picture was clearer it would be easier to ID.
