How concerned should I be about purple stems?

Purple stems or streaks of purple can be a few things more than likely its just the plants natural defense to intense light. if you look at a flowering canpoy you will find that leaf stems in direct contact with the light will turn red or purple this is complealty normal and is the plants defense mechinism against heat stress. Now purple of the stem could be a mixture of this and if its streaking its actully not a bad thing at all on the contary the plant is giving you a visible sign that she is healthy and getting them mobile nutrients around the plant. It could also be a slight lack or phosphorus when the plants are in this stage and look like yours they are going to require alot of P to keep ripening and fatting up. Hope this helps bro.

Could possibly be Genetics? If not genetics I see your temp Be's round about 19 Degrees Celsius try Turning that up to round about 25/27