What causes purple reddish upper stalks in late bloom??

[indica4mymeds]

I'm using 2-144*3,mars 2 400 in a 4 by 4 tent.
Didn't have any reddish purple stalks,radish colored,until week 6-7 of 12\12.
Using sensibloom,big bud for coco,sensizym, bud candy and voodoo juice 1st 2 weeks of veg,bloom cycle.
Coco\perlite 4:1 ratio.RO water only.Was feeding 100% big bud,50% sensibloom but not until 4th week or week 6-7 of 12\12 when it happened.
Only the upper main stalks are purple,peel back a bud its green under bud but anywhere light hits reddish purple.
Happened to all phenos on both C Skunk and medihaze but only upper half of plant,lower stalk is green.
No leaf burn,just 1-2mm on very tips of leaves.
I can't show Picts on tablet,but later on desktop.
Can LEDs be the problem??
It did get very cold before it happened,56-62 at night for8 days but purple has gotten worse?
Flushed and now 2 weeks later still purple but trich s are cloudy so harvest soon.
Any ideas so I can avoid next time?
Can slight nute lock cause upper main stalks and leaf stems to turn reddish purple or could I need calmag, even though big bud and sensibloom have calmag added???

 

[AKgramma]

Nothing wrong with purple buds! They come in all colors depending on strain. Cold temps will bring out the natural colors of maturing plants on new growth, but won't make the whole plant purple, at least not right away, and probably not as much with indoor gardens.

You don't need to fuss with it. Color change is normal for some strains, esp towards harvest. What is happening is that the Chlorophyll is beginning to die off, exposing its natural color, because the plant thinks its fall, (shorter days, colder temps) and will draw on its stored carbs to feed the buds.

 

[dbkick]

Could be what the gentleman/lady above said or it could be a phosphorous def.
The colder temps the user above noted may cause phosphorous def .