Although I have grown many plants in the greenhouse poinsettias are not my forte. So to answer your question, I contact the Poinsettia experts at the Paul Ecke Ranch. I spoke to Rebecca and she explained the process of developing Novelty Poinsettias which are the ones with unusual colors like purple or marble. To develop these types of poinsettias they manipulate the genetics of the poinsettia with different procedures like radiation or selective breeding. With this type of genetic manipulation there is always the chance that the plant will revert back to the original color which is often white. This is called a reversion rate and can occur in as much as 25% of the plants. Unfortunately when this happens there is not anything you can do to reverse the process.