purpleing of the steams is a genetic trait in most cases. it is the result of the pigment Anthocyanin in the epidermis tissue of the stems/leafs/calyx. its nothing to worry about in most cases. some phenotypes are prone to purple coloration no matter what and some will react to environmental stresses that trigger the response in plants where it otherwise may remain dormant, in which case the plant is most likely using it the same way the human body uses melanin, to block out spectrum of light that would otherwise be to intense and burn the plant (aka if the plant is purple its reflecting purple light).
Nicely put!
Ive only had one plant which was purple Kush. Stress actually brings the purple out more if the genes fore being purple is in its genetics. Other than that stress is the main source to bring any purple out in a plant. Most pure strains shown in pictures use lighting to make the bud look like its fluorescent purple. A drop in temperature is one way to stress a s
Purple gene producing plant to become more purple but a drop in temperature is nonetheless stress to the plant. Like changing the water used to feed a plants PH from 6.8 to 6.3. Causesstress. Id rather have a 15 degree temp drop than my PH get messed with though.
A temp fluctuation of 15F at night can express the purple trait that's non dominate in the strain/pheno. Temp drops ARE normal and the stress is minimal unless the lights off temps drop below around 45 and the swing is greater then 30+F.
pH stress, generally will not express coloring (unless the pH has increased the K being used by the plant). If you meant that the pH should be in the 5.8 to 6.2 range, IN HYDRO, you are correct.
Water soil with adjusted solution/water at 6.5 if you have severe pH problems with your water.
If the purple color isn't a genetic trait them your plant is under stress which can be something as simple as light getting in your flowering room w lights out. PH can also dress your ladies. I would make sure you have no light leaking in and check your PH. Never over feed or worse problems than you want to deal with.
Really? Stress is expressed in ways that will tell you whats going on WAY before any color expression does (colors that are recessive in the strain). These can be anything from orange to an almost brown to dark purples.
These above mentioned colors can be expressed with the manipulation of K in bloom. Up your K by 3.0 and this will express coloring. More so if the temps drop at lights off by 15F and this will not hurt the plant!
And Lastly,
purple stems are actually a sign of stress could possibly cool temps at night keep an eye on it.also your tips look a little yellow that's over feeding plants usually go dark green then tip burning so back off on the nutes if your e.c on run off is creeping up.
Enough on the purple, so I'll comment on the "yellow tips".
Yellow and even "dead brown" tips (say the last 1/8th inch) show that you are "pushing" the plant to the "brink" of over fertilization!
Not that you are "over feeding", but right at the edge of!
Unless you have a running knowledge and some good time under your belt growing, I would NOT suggest you push to this point unless you know how to back the plant off with out questions.
That being said, MANY nutrient makers feeding charts ARE running damn close to that max limit. That's what every newbie wants, to grow the biggest, baddest, most potent buds they can, and that's what many nute makers try and deliver!
Hell, some make wild claims and want you to spent buku bucks to go up to the "master" level and all they really do is raid your wallet! Just look at AN....
That strain you have looks a lot like it has some Chem Dawg in it. CD always runs purple stems!
Don't worry and keep going as you are.
It looks great!
Grow on dude!
hope you can help 
but I don't think u need to worry too much.
