Check out these fish info videos, maybe you'll find your answer: http://www.sutree.com/videos/Aquarium/200
unfortanetly I know nothing about fish...just like my salmon in my sushi :)
Are the parents gold? Cos if they come in different colours....
Or they haven't recieved enough sunlight yet, Or they are still not old enough to lose the protective colouring of fry.
Yes just checked the book - lack of sunlight most common cause. you can get colour enhancing food too.
You will find that the fish don't change colour at the same rate, it depends on all sorts of things, water temperature food supply etc. Its age or size doesn't matter. In my pond I've had fish 1 inch long and a couple of months old fully coloured and others 2-3 inch long and quite old that still haven't changed.