I have found that when I add new fish to my aquarium it can take 2-3 weeks at times for the fish to get adjusted to the new surroundings well enough to show their best colors. The more plants I have, the faster it goes. The longer it has been since I set up the tank, the faster it goes. I wouldn't be concerned until 2 weeks go by, then I would really study the fish to be sure they aren't just faded from discomfort, and if so, look for why that would be the case. A school - 6+ of one kind - seems to give me the best shot at contented, colorful fish quickly.