Which species are we talking about? I would say in general most species start to really shine around the 4" mark but again that depends on species. Coloration also heavily depends on diet and water perams etc.. Hard water and or poor diet make for some sorry looking fish.
What is the diet and water perams? When I had my G. abalios school on tap they looked like crap (hard water) days after being switched to RO they looked amazing including the smaller 3"-4" ones. ANy tankmates that might be stressing them out? At 5" you should be seeing a lot od red in the caudal fin and blue hues.