It depends on how long they take to change? Some fish will always remain the natural green colour, I have a spawning of Common Goldfish that after two years are only just 'turning' I have kept these fish as shape wise they are perfect and from a spawning of around 2,000 fry I have kept 10. The rest are in several pools around and about some have already changed colour to orange and some are still green with no signs of colour change. If I was to have kept the complete spawning and hoped, I would not have been able to have bred my show winners last year!!!

Colour Intensity depends on the line of the blood line of the fish and to an extent on feeding, and not on how long it takes to change colour.