My gold and pearl both looked lighter when they had growth spurts, and then the color (I am assuming shell thickness) caught up. They looked just like your pics. They were small though and grew very fast as my tank was heated, and they were from petsmart where all they were fed were clippings from plants that no one but canas eat and now they get food.
My mother in laws didn't look the same as she never fed them at all and let them get what scraps they could.
So I am assuming they grew very slowly. Someone correct me if I am wrong on this.
Like Jinkzd said, if they are eating and acting normal, I wouldn't worry. Lowering the tank temp can help. Do you give any calcium to them?