They color them up by using an array of different methods...using MT, Carophyll Pink, Red, Yellow, CR6, normal astaxanthin, and a few other hormoned based treatments. It is very common practice for coloring the fish to happen because color sells...if its dull looking, it wont sell in the LFS. People buy color. Put a AAA grade Brown discus in the same tank with a B grade hormoned PB based discus for a higher price...which one would sell first?? The hormoned PB discus of course!! It is super colorful and attractive. Even if it'll die in a few months time, it'll still sell. And thats the name of the game actually...especially with discus. A lot of brokers hormone the discus they sell so that the fish wont survive to live to adulthood, or if they do, arent able to breed. This is their way of controlling the market. They cant sell a few discus and then a year later have more competition with that same person breeding those fish! It'll kill their business. So, they color them up to make a fast sale to newbies for a good price...only to have them turn dark, and die a few weeks/months later. Broker will just claim that it is the customers fault with not maintaining the tank or doing enough water changes to keep them healthy. That is the #1 reason why discus have been known to be so hard to keep...because of past bad experiences with inferior quality fish from brokers/exporters who hormone their fish. Now a days, there are many more exporters out there coloring the fish with safe products like natural astaxanthin or carophyll pink that will not hurt them and will still let them breed. Unfortunetly, discus have been stuck with that past myth and will stay with it as long as people remain uneducated and unwilling to learn by trying discus again and seeing for themselves they really arent that hard as long as a few basic requirements are met. But as far as using coloring agents on tropical fish in general...they have to, so they can sell at a younger age and be able to make some amount of money with colorful fish.
-Ryan