From what I've heard and researched, discus that are not brown/blue based cannot retain/keep their color because discus do not create those pigments on their own. Therefore any red/orange/yellow discus need some sort of nutritional supplement in order to help them keep their colors. If, for example, you have a yellow/golden diamond and feed it foods that are intended to boost red colors, you can end up with an orange/red-orange fish. To boost reds, one can add astaxanthin powder to their custom BH mix. There are other powder supplements, but their names escape me at the present.
As far as blue/brown based fish are concerned, they typically maintain their colors without additional supplements apart from what is already contained in flake/pellet and other foods.
Adult discus are usually considered adult very loosely after they have reached 5-5.5" and larger.