Usually the change of color is stress.
Now for a few questions:
-How big is your tank
-What sex are your gouramis (this is determined by the shape of the dorsal fin the pointed fins are males and the rounded fins are females)
-what are your water paramaters (ammonia, nitrites, nitrates)
-are there hiding places for your fish (it is recomended that you only have one male gourami in a tank). That being said I have 4 gouramis in a 60 gal and three of them are male. They all get a long and do not chase each other, they even hang out together at times, but have enough places to hide and they can all claim their own area of the tank.
-how long have you had these fish, the may be claiming their territory.