The most common reason for them turning purple or red is temperature. Is it getting cooler at night? What is your avg daytime/Nighttime temps? I would say it could be genetic predisposition to purpling but that is unlikely in bruce banner. I have seen these colors from nutrient deficiencies, but at 10 weeks you are a week or two away from harvest. If I was you, I would simply water until chop. She looks killer. I have had a few very ugly bruce banners, but damn they produce and wow at the smell/flavor. I wish you good luck and happy harvests.