I've had mine for about 5 years now and she's tough as nails. I did some digging on the colour change and got everything from its age/sexual maturity to its based on diet, so I can't really say what it would be. Mine was about 4 - 4 1/2" when it changed and is around 7" now. Make sure you feed a wide, varied diet with a vitamin/HUFA suppliment. I do NLS pellet, PE mysis, Emerald Entre', Ocean Nutrition Angel Formula, hand chopped squid and clam meat and nori. I just mix it up from day to day and soak it in selcon. Mine will devour anything that goes into the tank, including any new frags I buy
The leathers that she grew up with are fine, but anything new in the tank is food.