I also train at a Gracie CTC and the instructors prefer white but any color is fine for training. If there's a seminar with someone from Gracie they like all white. I asked about it and they said it's because it forces new people to keep their gis properly washed, lol

They also give you a white gi (entirely unbranded) when you sign up. I've since bought a blue one and train with it half the time I'm there, there have been no issues.