I think I read somewhere that they get their color based on how acidic the soil is. If the soil is more acidic, the flowers are more pink in color. If it's more basic they are more purple in color. I think I read about it in Better Homes & Gardens. They have an excellent website where you may be able to find further info. Good luck!
Contact your local county extension office for details, but I was told that it has to do with the level of ph in your soil. I bought one years ago because it was white and after transplanting it became a light blue.
Login using the form on the top of the page to post feedback (if you are a registered user). If you have not yet registered, click here to do so. It's FREE!.