Guess it depends on the individual and diet. I have a 3 1/2 inch red who still has for the most part juvenile coloration. Spots, not a whole lot of red, and what-not. I've heard (and experienced it with bettas) that if you feed shrimp like krill that it will increase the amount of red pigments in the skin. With bettas I bred 2 white opaques and raised the fry on nothing but microworms and grindal worms and bred the same two bettas 2 weeks later and raised the fry on nothing but baby brine shrimp and powderized krill and the second batch came out with a lot more red in it's fins. Of course it may have been due to some other factor...oh wait, I'm still rambling. Anyways, yeah I think shrimp in the diet helps along with the red. I'm sure others will probably back me on that one. :
They should have their color (depedning on diet and conditions of course) when they reach 4-5 inches in about 6 months. I could be wrong on this but this is my estimation. Anyone else have a differing opinion?
My roomie has reds and all of them have basically no color even though they are about 4 inches in length. He feeds them only feeders. I have tried and tried and tried to get that jackass to listen to me about how to raise his piranha, but he won't listen. You'd think the fact that he got his fish at 4inches and they haven't grown at all would tip him off that he should listen to me but no. Sorry to blab on an on but I needed to vent on my stupid roommate. Each piranha has a different level of color it can reach but diet and care usually will provide you with a very red, red bellied piranha. Try shrimp it has my guys as bright as can be.
I agree with Joe: diet is very important for redbelly coloration: colorenhancing pellets, shrimp and krill are good food items to "max. out" the coloration.
Also, a happy (ie. stressfree and healthy) piranha will usually be more vividly colored than uncomfortable ones.
Mine started turning red at about 2", but my fish are pretty colorful 'by nature', so it seems: I didn't take any measures to enhance their coloration, but I guess mine are content.
btw: does anyone know if the amount of coloration is also genetically determined? I mean, do some reds have a brighter coloration than others, due to their genetic make-up?
The loss of color is usually associated with stress (ie they look really pale, sort of ashen-white). As they get older and larger the color will darken, as to when that's usually dependant on the individual fish. Probably shouldn't expect it really before 6" or so, that isn't set in stone.
My 3 have had good coloration for a long time and they are almost 5" now. I think it also has alot to do with their genetics. Some you may feed the best diet and have the best water and they still may not have good coloration.