Most likely, your gouramis are fighting for dominance. Gouramis come from the same family as betta fish. Males will fight each other in a small tank. Assuming your blue gourami is the three-spot variety and not the dwarf gourami, you will need a very large tank to avoid issues with males, and even then, it may not work if your male is overly aggressive. The highly colored male is the dominant fish, and he will most likely chase and harass the non-dominant male to death. Your other male is likely losing color because of stress and not wanting to draw attention to himself. My recommendation would be to take one of the males back to the fish store to return or exchange for some other semi-aggressive community fish.