It's very common for RT. They can turn black when stress or scare. Especially , when moving even to another tank in the same place will turn black . Try to keep RT in very clean water and do not over feed them. Try to make sure your water is clean with good filtration. And do water change only 10-20 % maximum every 2-4 weeks . Do not change water too often like flowerhorn . RT will turn themselves to red when very big size and few will turn red at 1 year age.