It looks like mild light stress or light burn. During flowering, you want to push for maximum light intensity, so it's pretty common to see signs like slight purpling on the stems (especially the side facing the light), some canoeing, or yellowing and burnt tips. The purpling happens because the plant produces anthocyanins, a natural pigment as a protective response to high light levels. It’s not a major issue, but next time you could dial the light back just a bit to avoid it.