It all depends on where the chromatophores are in the fishes skin.
They can be on the surface of the skin over the scales, deeper in the skin just under the scales, or even deeper toward the bottom layer of the skin.
Their density determines how colorful a fish is, and if that color lasts as the fish gets older.
As the fish grows the chromoatophores spread out, which can cause colors to fade if the chromatophores are only in the top layer of skin.
If they are in multiple layers the colors fade less as there will be fewer "thin" areas.
Fish from good bloodlines often have multiple layers of chromatophores and therefore maintain color better than mass produced fish.
Also, a good color enhancing diet can help enhance color as well.
There is a good article on this in the most recent edition of Practical Fishkeeping, although if focuses on Koi.
