Discus, assuming they are healthy and non-stunted, can grow in excess of 1" per month until they reach about 3.5-4" in size...then they begin to slow down to about 1/2" per month until they reach maturity, which then further slows down to even less until they reach and age of 2-3 years. So, to get a discus from 2.5" to 6" should only take about 6-8 months if everything is done right and you know how to do it....But on average, they will reach the 6" size range within 10-12 months. Feed them 3-4 times a day with nothing but a good beefheart mix if you want to max out their size potential. Also, water changes, water changes, water changes!! are key to growth as well. If you want accelerated growth, do no less than 50% water changes a DAY. The more the better.
As far as discus coloration goes...depends on the color types your discus are. For blue colored fish, mix in spirulina into their food (Beefheart is ideal for this). For red fish, mix in naturose or cyclop-eeze to make them brighther.
-Ryan