I have 4 WTFs. I find that when they hide in a dark area of the tank they turn brown, and when they are out in the light they turn green. I've had mine for over 5 years now and this has always been the case. I don't think you have anything to worry about unless they are lethargic and unwilling to eat. Do you have a UV light? These are really good for their skin and can promote brighter colors.