Probably more of a camoflage thing in young fish...just like many animals are when born...take deer for example they have spots on them at birth to help them blend in their surroundings better to not be eaten by predators.
because almost all africans look better when mature.
nah seriously tho its for camoflage a dull little yellow lab would be harder to see than a bright yellow with really Dark fins. its so they can hide from larger fish.
Heres an update on our 4 month old frys, http://www.aquariacentral.com/forums/showthread.php?t=122417&page=2 when released they were very pail with no black markings, at about 2.5 months they started showing some black traits, out of seven it looks like we have 2 males and 5 f-males.........