Could just be genetics... streaks of red/purple could be a sign of low S. Low S has other symptoms, so if they aren't coinciding, i'd lean genetics. Oh, it's one of the wierd ones that stand out too -- paling from petiole out rather than tip-in or interveinal, so 'other' symptoms would stand out in a discernable way.

Only worry about purple stems if you plant is looking sickly otherwise . on a healthy plant no big deal is the way i see it. I think its mostly due to light "stress"