it's a different looking kind of purple. not the purple you want your plants to be but yes there is a deficiency that can cause purpling, not just on the stems. not 100% sure right now but i think it is phosphorus
uptake of P is slowed to a craw in cooler temps. like 50's. but it's not the lack of P that turns the buds purple, it's the cold. the chlorophyl starts to die revealing the leaves TRUE colors. the green chlorophyl just masks the real color normally and you only see green all summer.
buy a purple strain like Urkle, or purple haze even my pineapple express is turning purple right now. night temps i'm keeping near 60.