Genetics can be the cause of purple stems.. Phosphorus and Magnesium def can cause purple stems... it can also just be a genetic thing.Genetics doesn't start purpling during flower.
^Most likely the cause. Your leaves do look good though, so P is probably present in the soil but interrupted uptake due to those temps. Once you go below 55°ish this can happenyeh it got about 45-50 degrees at night last week for 3-4 days