purple leaf stems

i am about to start my first journel. hope i can get some good advice and with as little as poss probs, hope to continue right thro and have some nice pics to post!
i am currently in week 4 with 3 SENSI AFGHANI #1, and 3 CH9 CITRAL SKUNK.
im going to re-pot tomorow so ill save the pics.
im using a 400w MH, and got a 600w HPS with added blue spec.
in pots with BIO-BIZ all mix. Seems to be great from seed with no need for the light mix which i here is pretty useless!
anyway ill be back tomorow with more info and pics.
p.s must be more stoned than i thought!
i was going to ask if anybody could tell me what makes the leaf stalks turn purple/red?
the guy in my local hydro shop said its a nutrient defect, and to flush. But i see it alot on other peoples pics and it doesnt seem to be doing much harm!

Deficiency or cold weather, can be both. I have the same thing at my plants.

so do you think its something i should worry about? I wouldnt want to start looking for one problem and creating allsorts of others if not too bad a thing in first place!

some plants have this trait, doesn't have to mean any problem
just grew out a northern lights, purple stems everywhere, she hits like a mack truck

im getting purple leave stems also, im in ohio and its getting cold at night.... low 40s! im sure this is from the cold but not sure, also not sure if it will hurt her!!

Thanks anyway dudes! I will let it be and keep an eye on things. Just noticed its only really one of the AFGHANI, the one i think that is going to be male! the others just have slight tints and all in same conditions so cant see it being due to cold. think its just a trait some plants get that others dont.
i will be starting my journel tonight and adding pics! just going to do some re-potting!