There are plenty of theories on purple stems and petioles. Often folks attribute it to P defficiency. Some think it is inconsequential.
I had many grows in a row that exhibited purple and still had fine outcomes. My current plants have bright green petioles and they do appear to be more vibrant than past grows. This grow i combined michael astera's fert with a mineral amendment blended by the lab that does my soil testing. Im not 100% sure though what is causing the change.