dark colored stems

I noticed some of the plants have stems that have changed colors and I wasnt sure what to do to correct it.

Ebb flow setup
feeding 3 times a day for 15 mins
GH Lucas formula 0/5/10 G/M/B
PH 5.8

lack of nirtogen probaly. flowering or veg??? how old???

i know sometimes the stems with purple or even redden when temps are too cold @ night.

TRU dat! wats ur day & night temps??

they look fine wouldnt change a thing at this point , looks normal and VERY healthy,
still would like to know how old Flower /Veg and Strain , if its a purp. then its really normal. even at early flowering ,
your not lacking N nute leaves look rich and nice shade of green .