If curds are starting to split, they are starting to bloom. You need to harvest them sooner.
Purpling is caused by too much heat or sun. Sheilding from the sun should help. I use an umbrella that screws into the ground. A tarp stung across the bed will work, not on the plants but above them. Ok to eat. The purple color is caused by the presence of the antioxidant group anthocyanin, which can also be found in red cabbage and red wine.
