What is the salinity? Freshwater? When GSP are small, freshwater is Ok and recommended by many. When they grow, it is good to very, very slowly increase the salinity over a period of several months. For more information on that, you will need to find someone with more experience on GSP.
They also produce a tremendous amount of waste, which is one of the reasons they need to be kept in a 30g tank. Another reason it is recommended they be kept in a 30g tank is that they are very active. If they don't have enough swimming room, they wont be able to gather enough oxygen with their gills.
The black belly could be parasite related. GSP are generally wild-caught, and many of them have parasites. I doubt he is changeing his color to match the inviroment (GSP are generally pretty bright in color).
That is as far as I can go with GSP. Hope this helps!