I've had a mandarin for a while and he changes colour to a pale grey/green every night. The first time I saw it I almost had a heart attack and thought he was dead. I now realize that it's normal behavior for a mandarin. Im not sure why they do this, but it does appear that when they "sleep" (become motionless) this occurs. Once the lights come on in the morning, it only takes a few minutes and he will get his colour back and will start moving about the tank again.
I would only be concerned about it if he is pale all the time.